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.... ;-)

It's interesting that no one has yet discussed the downsides to powerful amplifiers, as they are significant.

If a person is running an inefficient speaker (let's say 85 db. or less) or one that requires significant current due to low impedances in the bass or steep phase angles, then an amp with really stiff power supplies will, all else being equal, control the woofers better. When I write "amp with really stiff power supplies", however, I do not mean one that simply has a high wattage rating - as correctly noted above by several other posters, the fact that an amp has a high wattage rating does not necessarily mean that it has first-rate energy storage, e.g., Audio Research's 600 watt/channel monoblocks have less energy storage than CAT's 100 watt/channel JL-1 monoblocks.

Putting the issue of energy storage aside, there are two major disadvantages to high-powered amplifiers. First, it is very difficult to control a high-powered circuit without global feedback and virtually every high-powered amplifier therefore uses it. Global feedback sucks the life out of music by destroying three-dimensionality and giving it a closed-in, deadened quality, and can create highly complex distortion products. Most triode tube amps do not use global feedback, and well-known solid-state amps that do not use it include the darTZeel (Herve Deletraz HATES feedback), the Ayre monoblocks (Charlie Hanson HATES feedback), the new Rowland 625, a variety of Pass amps, and speaking of Sim, the Sim Audio W-5. Pass has made some very high-powered amps that do not use negative feedback, but most amps that eschew feedback are lower powered (the darTZeel and Sim put out roughly 150 watts/channel into 8 Ohms - the Ayre monos and the Rowland put out 300 watts/channel). For technical papers about negative feedback, see the Pass and Atma-Sphere websites.

The other downside of powerful amps is that they feature many output devices (by "output device", I mean output transistors in the case of solid-state amps and output tubes in the case of tube amps). All of those devices muck up the sound in systems that are otherwise high resolution because, among other reasons, it is very difficult to match output device pairs, every device adds noise, and they make the circuit more complex. For this reason, it is often said that the best sounding amp in an amp line is the lowest powered one. Most high-powered amps use a dozen or more output device pairs, while the darTZeel, in contrast, uses a single pair of transistors per channel. Circuit simplicity is indeed highly desirable - it's why many serious audiophiles put up with 8 watt (or less) per channel single-ended amps that use only one output device per channel.

If a person runs inefficient speakers in a large room and listens to electronic music (anything amplified and then recorded, like rock or pop), then high-wattage, brute force amps are arguably desirable. But if a person has reasonably efficient speakers (87+ db.), a normal-sized room, and listens mostly to unamplified acoustic instruments (classical, jazz), a high-powered amp will, relatively speaking, sound less like real instruments and more like an electronic reproduction.

So, how much difference does a more powerful amp make? It can make a huge difference - go listen for yourself.

If one listens to music at levels that come near the levels one might hear at a live venue, and that includes unamplified concerts, under powered amps can be subject to clipping during peaks. Fairly high powered SS amps without global feed back have been available since at least the 1980's. Some ss Class AB amps lines are able to stay closer to a Class A bias longer, as their total power output levels increase.
http://myhometheater.homestead.com/splcalculator.html
http://www.stereophile.com/content/threshold-stasis-sa12e-power-amplifier-page-2

"The other downside of powerful amps is that they feature many output devices (by "output device", I mean output transistors in the case of solid-state amps and output tubes in the case of tube amps). All of those devices muck up the sound in systems that are otherwise high resolution because, among other reasons, it is very difficult to match output device pairs, every device adds noise, and they make the circuit more complex."

Great sounding Coda Technologies S5, 50W/8ohm class A (no global feedback) has 60 output devices. As for noise it is exceptionally quiet with S/N=120dB.

State of the art Atmasphere MA-3 (also no global feedback) amplifier uses even more output devices (about 40 tubes per channel).

In both cases a lot of output devices doesn't "muck up the sound" but makes it better (at the expense of complexity).

Like someone stated earlier they make a difference for inefficient speakers with low impedence drops, they also make a difference with many multi driver speakers with complex xovers as the xovers are known to eat up as much as half the power before ever reaching the drivers