Is more amp power always better...?

The numerical value given for damping factor is 8 divided by the output impedance of the amp—the lower the impedance, the higher the damping factor.  A typical SET amp can have an output impedance of 2 ohms or more, for a damping factor of 4 or less.  This relatively high output impedance interacts with the impedance of the speaker, which varies with frequency, and thereby affects the frequency response of the speaker (the lower the speaker impedance, the greater the change in frequency response.  Thus, SET and other tube amps with high output impedances are best used with speakers with a rated impedance of 8 ohms or more, and with an actual impedance curve that is relatively flat.  Any damping factor above 8 is probably high enough to be irrelevant.

My thinking, a speaker having a true 89dB sensitivity to be used in a 10'x11' room can easily be by served by an amplifier providing 50wpc at 8ohms and 100wpc at 4ohms. 

My advice, search for an amplifier that mates best towards the presentation you desire rather than the power it delivers. Low watt SET amps an exception.  

Late again, but some thoughts.

Never buy just "watts." All watts are not equal. I would want something with good wattage, outstanding ability to deliver current, and the amp is not ’cool-sounding’ or ’warm-sounding.’ I’d want musc to sound like what is engraved on the record, NOT the amp adding its own colorations to the sound. Besides, the more ’personality’ an amp has, the more your music is going to sound alike, and nothing will surprise you any more.

People seem to think merely getting enough watts insures the best sound. That is a poor choice for determining sound quality. "Enough watts" does not mean that it can also make voices still sound ’delicate’ or ’poignant’ when required. Sometimes, the end result is less than one hoped for.

The qualities of sound are something a designer aims for when he/she is designing the amp, and not all amps have it. I’ve owned the Goldmund (Mimesis 9, Jadis Defy 7, Vtl 300s, Plinius SA 250, VAC Renaissance 70/70 and a few other of the ’super amps’ so I’ve realized that the amp that might be best in the high frequencies (the Goldmund) doesn’t necessarily do the ’quiet moments’ as well as, say, the Defy 7 did, or the 70/70. Or even the VTL 300s (no slouches!). But it took considerable experience (and money) to realize that no amps (or anything else!) ’had it all.’

The only component I have no complaints about is the ASR Emitter. It sounds similar (but not all the way) to my Antique Sound Lab Hurricanes, in that it does at least ppp to ffff, can sound delicate, poignant and heartbreaking when one is listening to a flute solo. It can sound bombastic, as when playing Stravinsky’s Rite of Spring. And it can rock out as in the Rolling Stones’ ’Gimme Shelter.’ And it does this while doing the unlikely solid state characteristic of having images with focus and true three dimensionality to the images, which is very hard for solid state to do. And it is a very high resolution device, which the Hurricanes, even with their nothing-short-of-astonishing realism, are merely ’good’ at.

So, one has to think about what qualities in sound are their highest priority, and then look for an amp, once you know what you value in reproduced sound.

You say you’ve decided to buy a solid state amp.

Here are some questions to consider:

1. How loud do you normally listen?

2. What’s the maximum loudness you ever expect to play your system at? For how long? (Like, are you hosting a party and need the music to play loud...

IMHO, the question you’re asking grossly oversimplifies things. Which can lead to misleading conclusions. I’d be curious to know what kind of music you like to listen to, what sources you prefer, and what you have enjoyed about your system or other systems you’ve heard.

I suggest you download a simple sound pressure level meter for your smartphone (assuming you have one). I use decibel X Pro. It’s good enough for my purposes, and I suspect, yours.

Use this to find out how loud you normally listen. You might be surprised to find you’re often using less than one watt. IMHO, 89 dB is loud! When I go to live shows the volume is often right around 95dB, and that is loud enough that I often use hearing protection.

If you’re listening to music below, say, 80 dB, it opens up many intriguing possibilities towards the low-powered end of the spectrum. A lower powered amp, thoughtfully evaluated, might be able to use a simpler circuit. Sometimes a simpler circuit can sound better. I prefer a very low parts count and feel fortunate that I can get by with about 1.5 watts. Higher powered amplifiers can sound different, but not necessarily better, unless I’m listening really loud.

Oh, one other suggestion - track down Markus Sauer's thinkpiece, which was published in Stereophile, titled "God is in the Nuances". Worth reading.

89db speakers can use a fair amount of power. How many watts depends on the type of amp you want.  My rule of thumb is, as an echo to a few posts above, a good amp should be able to double wattage between going from 8 to 4 ohms. This helps with reserves of power for transients.  Speakers have big magnets to move. 

This applies mostly to solid state A-B amps, which is mostly what I have had.  Tube amps are a completely different conversation. 

Some manufacturers under-report 8 ohm watts per channel so that they can show it doubling at 4. But get as close as you can, as well as the sound you like.