From Pass X250,5 to 350,5 : what to expect?

I am planning on going up from my 250,5 to 350,5 but first decided to ask if it is worth it.Are there any sonic benefits in upgrading to a more powerful amp in a ,5 line?As far as i know in the previous line (250,350)going up meant actually downgrading sound because 250 amp was sonically superior to 350 one.I do not really need more power but i heard that 350,5 is way better soundwise than 250,5.Is that true?Thanks for your insights on this matter.
If the circuit design is the same, which I think they are, I would think lower power always sounds better if it is sufficient power to drive your speakers in your room to the listening levels you want. There is a thread called "Power - Good or Bad" that has lots of comments on this type of question in a more general sense.
"What to expect?"

Nothing, unless you have really power hungry speakers in a big room.
Besides more ultimate power, many larger (compared to their lower powered siblings) class A/AB amps will provide more Class A power before sliding into Class AB.
Overbang, what speakers are you using?
Better dynamics. I strongly disagree that more power is a bad thing. Unless you only listen to quiet whispy music more power will give you better dynamics which equates to sound that is more like real life.
I use Revel Salons.
I have a X250.5 and driving Wilson Sophia 2 and didn't even think about upgrade to X350.5 ,which will not yield any significant sonic benefit IMO. However, new XA.5 class a series may bring you ultimate class A experience even with more difficult speakers since new XA series doubles output when load drops to 4 ohm.
You need sufficient power for dynamics, but your speakers determine whether or not a certain number of watts is a lot or a little. More power tends to corrupt the signal with more wire, parts, and complexity - so if you have sensitive speakers with benign loads you can take advantage of the inherent quality of lower-powered amplification. But yes, you do need enough power - no question, but that is a relationship between amp and speaker - with my speakers, 30 watts is a lot of power, with some speakers 100 watts is too little.
250 watts and the current drive into low impedances would seem to be plenty of power to deafen you with the Revel Salons, unless you have a really big room. I would think 110db would be plenty loud, the 350 would only increase the output to 111-112db, I would think you would be running out of the room before you could tell the difference. Again this is assuming the 250 and 350 have the same basic circuitry, dam[pening specs, and drive into dipping impedances.
Given the impedance curve and sensitivity of the Salons, you have all the power you need.
Having gone from 200 to 300 watts with both Levinson and Pass amps, driving fairly inefficient speakers, I can confirm that more power is better. Better dynamics, better headroom, better low-end control. The only sacrifice for these considerable benefits is the one from your wallet. Well worth it though.
You will get better sound. I the case of Pass Labs .5 amplifiers, the X350.5 is regarded the best among the stereo amps. Something magic is present with the X350.5 but it will come at a cost. The sheer mass of the construction is maybe the answer to you tweaking guys out there. The power you are using for your cuddling moments with Elvis or miss Krall is subtle, never exceding double digits. The obvious difference between the amps is power rating but since most playing is only a fraction of the rated power actually used, the answer has to be somewhere hidden from ordinary wisdom.
>Having gone from 200 to 300 watts with both Levinson and Pass amps, driving fairly inefficient speakers, I can confirm that more power is better.<

That might be true for your speakers/amplifier, but in this case there will be no difference.
Well, being a dealer I can A/B them. I have and this is what I see: Deeper bass, hard to believe but there is way more control, seemingly endless EFFORTLESS power, more transparent (hard to believe), more open on the top and sweeter sounding. Basically, you get more of everything. If you like the Pass sound you will get more of it
Fatcataudio, what speakers did you use for your evaluation?
Proac's and AAD 7001's.
Well, I can't argue with what you experience, it just seems hard to believe that going from 250 watts to 350 watts can make any noticeable difference with fairly efficient speakers with smooth impedances (which I think most Proacs are). Now it may be that the Pass 250 and 350 have fundamentally different circuit and/or parts, but I did not think that was the case, just more watts, and in this case just enough additonal power to play 1db louder.
I have to agree with Ingvar. I don't think that sheer watts is the reason for a difference in sound. It must be that a different circuit or design or components are used to produce the higher power. Or like so many other things in our whacky hobby, there is something we are yet even aware of yet that has an affect on the sound.
There is a difference in the design, the X350.5 use no negative feedback in the circuit. It also runs pure class A to 50 watts versus the 30 of the 250.5.
Well the NFB would be a signficant difference for sound quality and that might explain the difference some have heard, although I suspect the 30 versus 50 in Class A would not be to relevant with the Revels.
Fatcataudio, do you expect much of a difference between x350.5 vs x600.5 vs xa160 for either a Wilson Watt Puppy 8 or a Sonus Faber Elipsa or the like? Pass Labs thinks the xa160 should have enough power to handle those speakers. I have my doubt. Yet that pure class A of the xa series is tempting at the 80 db sound level.
spatine, what efficiency and nominal (and minimum) impedance of your speakers? You will be very surprised at how little power you actually need if you listen at normal listening levels. I actually calculated the power my speakers needed at varying volumes, listening distances, and peak music levels. I was surprised to just how little power is required.
owned 350.5, 250.5 and 150.5

350.5 is smoother and also has more bass impact, 250.5 is brighter and bass is a little loose, transparency is about the same. Tried 87dB and 90Db speakers. I rather have 150.5 than 250.5
Spatine, I would go with a pair of the new XA.5 series as they double power as the ohms half. This may not be an issue, but since the Wilsons look like they are 4 ohms you would be doubling power, so in my book it would be a nice benefit if needed to get the XA.5 series. I would suggest the XA60.5 or the XA100.5. I don't think the added power of the XA160 is needed. please let me know if I can help at all.
I just have to ask if the 350W X350.5 is needed (vs X250.5) for Wilson, why do you think the 100W XA100.5 is enough? Secondly don't the XA and X.5 series (and not just XA.5) double their power going from an 8-ohm to a 4-ohm? Thirdly I heard the rumor that XA.5 is just a quick re-design of the XA. In other word, any changes would be minor given the cost factor. Is that true? Fourthly, we are talking about 75 db for rock music and 55-85 db for classical music. Are you sure that 100W is enough. Thank you.
spatine, what is sensitivity of your speakers and its nominal and min impedance? If I have this info I can calculate for you the max power you will need. Also, how far away do you listen?

The XA does not double down power.
The XA.5 series DO double down in power. The XA series do not. There was a mild re-design to allow the doubling of power. It is sonically insignificant, but if you want or need more power on tap the doubling down is a bonus.

Let's let Taboooe do the calcs for your power requirements. As for the amp size, I never recommended the X350.5 over the X250.5 or anything else, I just chimed in and said it sounded better to my ears for the reasons I listed. I believe the XA60.5 or XA100.5 would do just as well and offer things the regular series do not such as righer midrange, more liquid highs. At the end of the day, do you want pure class A or hybrid A-A/B?
Tboooe, since I am replacing both amplifier and speakers, info on my existing equipment won't do any good. I am going to go with Wilson Watt Puppy 8, thus rated sensitivity is 92 db, and impedance is 4 ohms. Distance between the tweeters and my ears would be 7 feet. Rock music is typically played at 70 db. Classical music goes anywhere from 55 to 85 db, but then it does not have as high a demand on the power hungry woofer.
spatine, assuming the sensitivity you provided is at 8ohm, here is what I calculated based on your numbers (these are only best estimates):

at an average volume of 70 db assuming a peak of +15db, you will need 0.03W at 4ohm and 1.0W at 85db

at 81 db average you will need 0.5W at 4 ohm and 16W at 96db.

If you speakers dip down to 2ohms. then double the power.

As you can see, you really do not need a lot of power. For you to use around 250W of power at 4ohm, you will need to be listening at 108db!

In opinion, most people do not need high power amps. However, as it has pointed out, sometimes higher power amps have better circuits or designs which produces higher quality power at lower output.

Note that these calculations were obtained from a spreadsheet I developed which has some assumptions, approximations, and estimations. For example, I assume it takes double the power to increase the volume by 3db. I also assume that the volume decreases by 5db for every doubling of distance from 1m.
Tboooe, I think all your assumptions are right and your conclusion regarding the need for power is right - we generally need a lot less than we think. I would only argue that given the same basic design approach, the lower power version of that design will sound better at a given volume level if the amp has enough power to reach that volume level with a given speaker. In the case of the 250.5 versus 350.5 comparison, the more powerful amp may sound better in this case if it is a no negative feedback design and the lower power amp isn't.
Thank you for the calculation, but it's pretty obvious that theory does not match reality.

As to the whole thread it does not seem that there is agreement either. The punch line actually reads 350W is preferred over 250W, yet 200W is as good as it's going to get (assuming the doubling of power for a 4-ohm). To be exact, even 120W is plenty. This does not, I think, help the originator of this thread or myself. The whole discussion is strictly about Pass Labs. So to make sense out of the power vs performance question, design differences between these amplifiers of the same designer must be pretty drastic. It's too bad that Pass Lab does not have enough dealers for a convenient audition. By default, I begin to lean more and more toward Ayre MXR by the week.
The design differences between and the X.5 and XA.5 are quite "drastic". And pure Class A watts are not the same as A/B watts - in part because there is more to power and sound levels than just watts. By the way, Ayre is also first-class SS amplification - can't wrong with them either.
With all that being said, I would still recommend the XA60.5 or the XA100.5 depending on budget and need for the pure clase A sound versus the A/B. With the Wilsons, again because they go down to 4 ohms I recommended the XA.5 series b/c they can double power. There are times when the ability for an amplifier to provide more power can be significantly appreciated. Even though by the calculation the needed watts to acheive a spl are low, having more watts does offer more headroom and this comes across as control and dynamic differences. Thus the recc for the XA.5s that double down.
The X series also doubles power, always has. But the XA is a better sounding design if sufficient power. The XA100.5 is wat more than enough power with the Revels for 99.99% of the time is most rooms.