Yeah, I guess like 25-30 watts. That's my gut instinct, anyway.

The thing is that with tube amps a lot of that power will be distorted, while all of the solid-state amp volume will technically be 'clean.' The reason I say that is that if the SS amp is actually clipping, it would be square-wave, nasty-sounding, and not a planned part of the amp performance. The clipping in a tube amp is a big part of the amp selling point.

Also, tube amps naturally add compression which has the effect of sounding louder. So if you want to look at it this way, a tube amp played at the same level of distortion as a SS amp would probably be roughly the same volume, but they're designed to operate in very different ways.

I probably just muddied the issue, so feel free to ignore everything but the first two sentences.