How Much Difference Does a More Powerful Amp Make?

06-21-11: Pani
Since you are moving within the same make and lineup of the amp you will probably not have "inferior" watts.

not necessarily true, AP. the bigger amp *could* have inferior watts; it could have superior watts. It's 50-50 without knowing any more about these 2 Sim Audio amps.
There are several such examples in the industry: the vintage Krell KSA-50 was acknowledged by many users to be a very sweet class-A sounding amp, while the bigger amps in the KSA series such as the 160W or the 200W monos did not have that sweet class-A sound. These bigger Krell amps were really very good but didn't have the 50W amp's sonics. Same could hold true for these Sim Audio amps....

With your speakers it is a big thumbs up to go higher in watts.

based on what???? where's the info or data to support this? have you been reading the manuf website & that's how you know??

What you will most probably hear is a more resolute, disciplined performance because of the grip the amp will have on the drivers. The sound might be bigger as well but it will be more graceful.

based on what?? where is the information or the data to support such a statement. it's pure conjecture on your part....

To Level8skier (from Level0skier!): the question you are asking does not have a definitive answer thru mere written discussion on this forum 'cuz there isn't enough data to supply a good/correct answer.
If you increase the wattage to 250W from 150W, you increase dynamic headroom by 2.2dB. Increasing dynamic headroom is always a good thing for music - you have more peak power that can be delivered into your speakers when there is sudden need from the program material.
That said, another aspect of power amplifiers is its ability to deliver current into the load. That's what sets up the instantaneous voltage in the drivers which creates the amount of pistonic action in them. If your stated wattages are correct, the 150W amp uses a 35VAC transformer whereby the DC rails are +/- 49VDC & the 250W amp uses a 45VAC transformer whereby the DC rails are 63VDC. If the only thing different between the 150W & 250W amp is the power transformer, then, the 250W amp has less load current ability (vs. the 150W amp) 'cuz at higher DC rails, the power transistors can source/sink less current (for a set bias level) according to the SOA graph.
OTOH, if Sim Audio increased the power transformer AND increased the number of output power transistors, you have a higher wattage amp which will have the ability to deliver a higher load current. My understanding is that higher load current is better because the amp is able to maintain is damping factor (DF) better ('cuz as load current increases, the amp output impedance stays relatively unchanged hence DF remains relatively unchanged). In such a case, the 250W amp is a better amp.
You have not told us which way Sim Audio went with the 250W design. Maybe you do not know? Maybe you will never know? Depends on how much Sim Audio wants to share with you....
If you are unable to decide after reading all the available info &/or talking to Sim Audio, it might be better to audition the 250W amp in your house or at your dealer. That would be the best route. In the mean time, all of us can speculate galore for you.... ;-)

06-22-11: Raquel
......
First, it is very difficult to control a high-powered circuit without global feedback and virtually every high-powered amplifier therefore uses it. .....

Raquel, I believe that this is *not* a true statement. I think that you are confusing power amplifier gain with power amplifier (output) wattage. They are different. You need global negative feedback when you have many cascaded gain stages (wherein the power amp has a lot of output to input voltage gain) so that you can keep the entire design stable (oscillation free), lower in distortion & lower in overall noise.
To design a high wattage power amp you need to design in the correct transformer size, the correct number of output devices to handle the output current, the correct size & quantity of power supply caps to handle the charge being delivered to the load. This can be done using minimal number of power amplifier gain stages (for example, Pass does this often with just 2 gain stages hence he can avoid global negative feedback & still deliver to the market a X600 beast of an amplifier).

Also, you use the term "high-powered circuit". what, according to you, is a "high-powered circuit"?? you probably mean 'high wattage circuit'? Might be better to use the correct terminology to avoid ambiguity esp. if you are sharing some of your technical views on a subject. Just FYI.

06-23-11: Raquel
Bombaywalla:

I was trying to keep it simple to make a point.

ok, thanx for the clarification. I now know where you were coming from.

Most amps, powerful or not, have anywhere from three to five gain stages (very few have two),

OK, if you say so. i will not contest this as I certainly do not know this for a fact. I have not popped the lid of various amps nor studied their schematics to know any better. So, i'll take your statement at face value....

and we all know that using global feedback with such designs makes the amp builder's job a lot easier.

there was never a contest here - we are in full agreement.

I do not agree that feedback ultimately lowers distortion

I think that you should! ;-) any electronics book that discusses feedback will discuss 'feedback effect on distortion' & you will see mathematical equations showing that negative feedback does reduce distortion.
Having said that, in audio where we are talking about human hearing which is very sensitive to distortion components, I tend to agree with Pass' following statement (taken from your post):

Negative feedback can reduce the total quantity of distortion, but it adds new components on its own,...

I believe that you are in agreement with this statement as well.
this might be a better way of stating the distortion, negative feedback relationship.