tube amp wattage questions/psb golds

Assuming that both measurements are made in the same manner and at a low THD limit, say 0.05%, - then the answer to the last part of your question is "100wpc". The biggest reason why tube amps with less power are acceptable is that the distortion in a tube amp is both more gradual and of different harmonic content than the distortion in a solid state amp. The tube sound is mainly from these distortion properties (and I expect from the output transformer where one is used). One would be hard pressed to tell a difference between a solid state amp and a tube amp amplifying a signal at a setting where both have the same thd.

There are certainly high wattage audio tube amps but they tend to be big fellows due to the transformer requirement. The higher the wattage, the bigger the "iron" needed. (If you want to talk about sheer power, tubes are by still commonly used as output device for TV and radio transmitters for 10,000, 50,000 watts and more.)

An undistorted watt from a tube amp is the same power as an undistorted watt from a transistor amp with the same rating.

Where you start seeing the difference is how the amps behave when driven above their rated power into clipping territory. A tube amp clips more gracefully (from a sonic standpoint). It acts as a natural limiter for the peaks while increasing the apparent average volume level. A transistor amp tends to give a nastier sound when it clips. So even though the average volume level is within the power range of the amp, the nasty sounding peaks make it clear the amp has run out of oomph and needs to be turned down.

Add that all up and it "sounds" as if the tube amp has more power. A common rule of thumb is that a tube amp might sound "twice" as powerful as a transistor amp.

All of that said, there are good tube amps and bad ones. Similarly, there are good and bad transistor amps. There are also a large number of solid state designs using different output devices in a variety of topologies. Speaker impedance and reactivity are also big issues. You may find that a particular speaker is not a good fit for a tube amp and sounds much better with a particular type of transistor amp.

So, generalities are nice, but your best performance is going to come when you get a good match between amp and speaker. That is the far more important consideration.

so a 25wpc tube amp wont drive my psb gold any better than a 120w ss amp. i dont have any place close to me to audition any amps at all. how could i possibly make a educated decision on what sounds good without buying it?

Flyinjz sez: how could i possibly make a educated decision...

Welcome to reality. Even professional reviewers only get exposed to a small percentage of the available equipment out there. The best you can really do is narrow the field to what seems to be eligible candidates and go from there.

The first step is to see what others are successfully using with speakers like yours. That means reading reviews and forums. I'm a Spendor guy and haven't listened much to the PSB line beyond some cursory auditions. However, with Spendors, you'll see a lot of people using solid state with the new "S" series but many of the owners of the "classic" prefer tubes.

From there you can see what your local dealers have available. Some brick and mortar dealers will allow home auditions.

Another option is right here on Audiogon. If you shop wisely you can typically buy used equipment, try it out, and then, if you don't like it, resell it for about what you paid for it. If you like it, then you're set and you paid a lot less than retail. Not a bad way to put a system together if you are prudent in your dealings.

The biggest issue for you is to spare yourself the physics and understand that the "tube watt"simply sounds different that soynd is perferred by many,myself included . The 120 watt SS is a really good high current amp it will distort less than any 20 watt anything amp SS or tube.
The problem is how the speaker behaves as was said above. The main point is to understand that tube amps distort more with speakers that drop their impedance. Believe it or not--- even low sensitivty speakers but not very low impedance, can be driven by a low watt tube ampIn the case of driving a low sens. only the big power supply tube amp is used to keep it linear . Braodly speaking that implies big trannies.
My 60 watt tube amp weighs about 80 lbs. with the majority of that weight coming from the transformers. I have yet to clip it and I like it loud . My speakers are not super high sensitivity they are 88 Db/W/M.
But and this is where the crux of the matter is . They hold their impedance close to the stated 8 Ohms no matter watt. Yes the impedance drops but not very low they claim no less than 6 Ohms. Some tube amps have transformers that are engineered to play speakers at 4 Ohms , like my 78 watt per channel tube mono blocks.
You do have to consider the impedance and sensitivity in building a system. There are tube amps rated much higher than my 60 wpc integrated that will distort more quickly as you turn up the volume. And if you are contemplating electrostactic panels it is very difficult to find a tube amp to tolerate a 1 Ohm impedance. Thus think about a speaker in terms of and the impedance and sens. before trying to use a tube amp.
The speakers you have are going to be a bad match with a 20 watt amp unless it is a really good amp.
Do yourself a favor and try a tube integrated with speakers that keep the impedance near the rated 8 ohms The sound of tubes is really magical if you can get true advice in helping to select the components tubes sound different. To me no SS amp will sound as good with my JM labs Electra 938 speakers because. I tried a few and was frustrated for years until..... I finally bought tube power amps and I was..... blown away it is now Nirvana every time I play them. BTW they are Opera Consonance "Cyber 800s". If you don't they sound good, then you are a SS person period.
Nothing wrong with that but this is not a question of watts against watts it is much more like comparing apples and oranges.