Tube vs Solid State - Comparable Output?

You might like to look at the August Stereophile, which has an article on this.

In regards to the ability to deliver current into a low impedence, that is THE weakpoint of a tube amp. Classic(not usually in the real world) solid state amps doubles power as the impedence is halved(that 200W could act like a 400W amp). A tube amp puts out the same power(unless it is an OTL, in which case it would put half as much power into half the impedence). Power ratings(the ones that we talk about) are usually specified at 8 Ohms. And with a speaker that is not a difficult load, the analogy you stated is usually accepted. But with a difficult speaker, the tube amp will never do it justice. Now, if you use one of those monsters from VTL, etc. with 16 or 32 output tubes, you can drive even a difficult speaker. But they are REALLY expensive, and get even more so when you retube(every 3500 - 5000 hours). Assuming you could buy a matched pair of 6550s(discounted, via the internet) for $60, times 16 = $960! But, if I was using a tube amp, I wouldn't not give it a difficult load. There are plenty of easy to drive speakers around today.

Don't use a tube amp on a speaker of low impedance, PERIOD. Neither the amp, nor the speaker will sound the way it should. I know some tube amps that appear to drive 4 ohm speakers well...until you listen to them on a speaker with an 8 ohm or higher impedance. Your friend sounds like an idiot.

I have heard that a low impedance speaker, like an electrostatic, it will eat tubes like there's no tomorrow. On the topic of equivalent power. I've had 100w tube, 160w tube, and a 185 w solid state amp in my system. I can attest that the solid state is MUCH more powerful in terms of controlling the speaker. They are not even close. I do not drive my speakers to high levels, so I cannot comment on clipping points. I also note that when people talk about a tube amp having good bass, it is in terms of a tube amp. I had two high quality tube amps, and neither had even a shadow of a solid state amps bass control. The amps were AR vt100m2, Sonic Frontiers SFM-160's. For solid state I have , a McCormack DNA1/AGold. The SFM-160's actually had internal jumpers to that you could set for 8,4,2 ohm loads. The vt100 had 4 and 8 ohm terminals. I have also had my old 125w Denon receiver hooked up to the speakers, and although the sound quality was lower, the bass was still more powerful than either of the tube amps. I've driven Vandersteen 2's and 3s (8 ohm), and virgos (4 ohm) with these setups. The vt100 seemed to be a little wimpy with the vandersteen 3.