Why is an XA30.5 a bad match for Revel Ultima Salon II speakers?

Hi sboje,
    I haven't heard these combinations, but overall, I can't imagine why your Pass would not drive either of these speakers.  The problem is that these speakers are both 4 ohm relatively low to medium sensitivity (86db) and both require an amp with very good current capabilities to drive them... that's the catch.... Most big amps have big power supplies which gives most of them the capability of the current needed to drive such speakers... but if I recall the XA30.5,  it is rated Class A to 30 watts,  but its actual power is more like 130 into 8 ohms and 190 into 4. I'm not saying that some big powered amps out there wouldn't be better, but  Unless you are a loud listener that drives em hard,
I suspect that it would drive them just fine.  I'm sure someone out there would give a more solid answer, but that's just 2 cents worth, Tim

After having a Parasound A21 amp and JC 2 BP preamp I suggest you keep the XA30.5. Once you have the Salon 2's you will know if it has enough power for your listening needs. If so, you saved some money. If not, keep the XA30.5 until you have the funds to get something of similar quality that has more power. Just my two cents!

good question! The specs for the XA30.5 are pretty good (not surprising since it's a Pass Labs product):

Description: Solid-state stereo power amplifier. Inputs: 1 pair unbalanced (RCA), 1 pair balanced (XLR). Outputs: 2 pairs binding posts. Rated power output: 30W into 8 ohms, 60W into 4 ohms (both 14.8dBW). Maximum output voltage: ±35V. Maximum output current: ±20A. Voltage gain: 26dB. Input sensitivity: 0.77V at 26dB gain. Frequency response: 1.5Hz–100kHz, –3dB at 1.5Hz, –2dB at 100kHz. Signal/noise: 150dB at full power. Distortion: 0.01% at 3W, 0.1% at 30W. Input impedance: 30k ohms balanced, 15k ohms unbalanced. Damping factor: 150 at 8 ohms. Slew rate: ±50V/µs. Power consumption: 238W.
Read more at http://www.stereophile.com/content/pass-labs-xa305-power-amplifier-specifications#pFvKLDoXoFb0JoE8.9...

with it's ability to output +/- 20A my calculations show that it should be able to double in power for each halving of the speaker load impedance (I expected nothing less from Pass who's been a major contributor to writing the book on how to make a class-A amp).

But, let's look at the Revel Ultima Salon II impedance & phase plots (Fig1 in the link below):
http://www.stereophile.com/content/revel-ultima-salon2-loudspeaker-measurements#fm6YgtoIdAOUp14u.97

IMO a tough impedance & phase plot - large swings in phase swinging from capacitive in the deep bass to inductive in the upper bass & lower midrange, rather low impedance (4 Ohms) in the bass. This is a hard speaker to drive for any amp. Definitely bring a high current amp to this party (which the XA30.5 seems to be).
The thing to remember is that with non-zero phase angles, the impedance is actually much lower than what is plotted. For example, at 50Hz, the phase angle is approx -20 degrees. My calculations show that the resistive part of the speaker impedance (which is the part used by the amp to create a voltage to drive the speaker cone) is actually 3.57 Ohms (not 3.8 Ohms as plotted in Fig 1). Having a reactive component to the speaker impedance exacerbates speaker impedance i.e. makes a low impedance appear lower to the amp. Thus, the amp struggles more as it has to dump current into a (further) lower impedance.
For a moment forget the brand & model of your power amp.
A 30W power amp (no matter which brand) can output 1.9A into 8 Ohms.
A 60W power amp can output 3.8A into 4 Ohms
A 120W power amp can output 7.75A into 2 Ohms.
All of the above ASSUMING the output voltage does not violate the max output voltage capability of the amp + the AC power xformer is capable of delivering this sort of current.

Next, how much power do you need to generate the SPL at your listening position?
Assume you listen at 3m (10') from the speakers.
The Ultima Salon II is 86dB/2.83V/1m from the Stereophile review.
Thus at 3m the SPL would be 86dB - 6dB - 3dB = 77dB. This includes the detrimental effects of sound absorption due to walls, furniture, drapes, etc + the positive effects of having stereo playback. Essentially these 2 effects wash out.
So, 1W gives you 77dB SPL at your 10' away listening position
10W will give you 87dB SPL at 10'
100W will give you 97dB SPL at 10'
200W will give you 100dB SPL at 10'
400W will give you 103dB SPL at 10'

103dB SPL for transients is not very loud - it's decent tho' but on the lower side. If you want your listening experience to be realistic & you want to get the feeling that the music flows effortlessly, you are going to need atleast 400W of power.
With the Ultima Salon II having an impedance in the 3 Ohms range your XA30.5 is going to operate like a 70-80W power amp. If you reference this to the above SPL table you'll see that you'll get an approx 97dB SPL max out of it. This is not going to be very loud nor realistic. It's going to feel that the amp is reigned in & that the music doesn't have that sparkle or that effortless flow.
For this speaker if one wants realistic levels of SPL for the transients then power begins to increase rapidly, as you can see. 
That's why the XA30.5 is not a suitable amp for the Revel Ultima Salon II speakers.
Hope this helps. FWIW.

Thanks for the lengthy reponse Bombaywalla!

Most of my daytime listening is around 80db to 85db according to my ratshack meter's C weighted average mode. I really have no idea what the true peaks would be. I think most music I listen to wouldn't have anything too extreme. Listening to the 1812 overture is a compromise though... I like to turn that up so that I can hear the quiet passages, but my amp is running out of steam for the canon shots.

Having realistic playback levels sounds like a worthy goal, but to give you an idea about my hearing sensitivity, I often wear musician earplugs to play my grand piano because it's too loud for me.

I feel like my system is most coherent and enjoyable at the 80ish db level. Brass and other instruments are really vibrant, and even startling at low volumes.  I'm not sure if that's primarily because the amp has an abundance of headroom at this level,  because the room gets overloaded with slap echo and wall vibration at higher volume, because humans have a different hearing response curve at different volumes, or if it's a combination of all three or other factors. I'm really pleased that my system sounds very dynamic to me even at 70db, and my fiance probably appreciates that at 2am even more than me.

Do you think the dynamics would take a turn for the worse at 80db in comparison to my current M106 speakers?

Is it harder for an amp to send a 25hz signal to a small speaker like the M106 that can't reproduce that, or a speaker that can? Does the crossover actually trim that out of the small speaker and make it easier?

here is a link to the imp & phase plots for your Revel M106 stand mounts:
http://www.stereophile.com/content/revel-performa3-m106-loudspeaker-measurements#ljf3ojqamBz1KzED.97

tough imp & phase plots once again...
And, here is a synopsis of its specs:

http://www.stereophile.com/content/revel-performa3-m106-loudspeaker-specifications#usPtaevBJxpkUyqE.97

with a -3dB at 59Hz, it's easy to see why the 1812 Overture canons are lacking. Your stand mounts just don't have the capacity to reproduce such a low freq. When we say -3dB at 59Hz the SPL/volume is half (-3dB) at 59Hz. The SPL is probably back to 100% at a much higher frequency such as 90-100Hz. So, essentially, you have very bass coming from your M106 speakers & I think you wrote that you use a Velodyne sub - I can see why. 
The XA30.5 "sees" the x-over of the M106 & this x-over is providing a high impedance to the bass frequencies so the XA30.5 is probably sending very little power to those low frequencies. IOW the XA30.5 is probably not taxed much at these low freq due to the M106 design.

For the Ultima Salon II speaker, it's a different ball-game. The -3dB freq is 23Hz & 23Hz is in the deep bass region (unlike the M106). Now the XA30.5 will "see" the Ultima Salon II x-over & this x-over will demand lots of power into these low bass frequencies. I expect the XA30.5 to run much hotter temp wise as it tries to deliver power into these low frequencies & the XA30.5 will get taxed much more than it ever was with the M106.
So, yes, I expect the dynamics to worsen at 80dB SPL with the Ultima Salon II.

One more thing to remember (that many forget) - even if the Ultima Salon II are 23Hz capable, does not mean you will be able to reproduce 23Hz in your room. Based on your room dimensions you will be able to support a certain min frequency.
Here is a room modes calculator that will show you what the min freq your room can support:
http://amroc.andymel.eu/?l=19&w=17&h=9&ft=true&r60=0.6