tube watts vs solid state watts

Watts is watts! But since you can drive most tube amps well past clipping (into serious distortion) without hurting your speakers, many tube users routinely clip their amps to high distortion levels to get a little extra volume.

If you do that with a solid-state amp, the odd-order harmonic distortion will grate on your ears, and you'll likely burn out your speaker's tweeters. This is why solid-state users prefer to use amps with more than enough power to get the job done, while tube users take advantage of the tube's soft clipping nature and many times use an amp that is way underpowered to drive their speakers. I guess they don't mind/notice the compression, and they welcome the extra fuzziness of the distortion.

And I don't know of any tube amps that have 3 to 6dB of headroom. Actually, not too many amps of any type have that much headroom.

Actually, most tube amps run into difficulty driving low impedance loads and put out less power at 4 and 2 ohms than most equivalent solid-state designs (many of which double power into 4 ohms and then double again into 2 ohms). So if you have a well designed ss amp rated at 50Wpc into 8 ohms it could put out 100Wpc into 4ohms and up to 200Wpc into 2ohms. A typical tube amp rated at 50Wpc will put out nearly the same 50Wpc at 4ohms and much less power (maybe 25 watts) at 2 ohms. That said, a few tube amps are specifically designed to drive low-impedance loads more efficiently. I believe that VTL has a few models optimized to drive 4-ohm loads.

Additionally, tube amps traditionally have terrible damping factors which is why their bass is loose and indistinct compared to decent solid-state.