Amplifier Power tubes - important to sound?

Yes, power tubes are important, and not just for "tone."  I recently tested a variety of output tubes in my Williamson amplifiers.  Just to give a few examples, the Gold Lion KT66 produced the most power before passing 1% THD, 22 watts.  The Chinese TAD KT66 produced only 19 wpc at 1% THD.  Vintage 807s also produced only 19 watts before clipping, but at only .19% THD!  Dave Gillespie over at Audiokarma is an expert on ARC amps and he says that the Gold Lion KT88 is about as close to the GE 6550 as you can get, in terms of power output and distortion.  I've found the same with a number of small signal tubes, including 6SN7s and 12AX7s  In general, the NOS tubes perform better than most new manufacture tubes.

@jasonbourne71 I’m sorry, what are you talking about? It’s very easy to substitute an 807 for a 6L6 or KT66 with an adapter socket you can buy on eBay. You just have to be careful to observe the lower dissipation rating and keep the plate and screen voltages at 400VDC or below. The 807 is pretty much a 6L6GB with slightly higher max ratings. My amps will accomodate KT66, 6L6GC, 5881 or 807. And the 807 uses a *five* pin socket. The 1625, the 12 volt version of the 807, uses a seven-pin socket.

Just to be clear, I was certainly not recommending anything other than the correct tube type for the OP’s amplifier. I was simply pointing out that the quality of the output tubes can make a difference, in terms of both power output and the amount of distortion they produce.

"Curious, I read herein that the feedback level related to the amplifier is important to consider for purposes of evaluating whether it really makes much of a difference to tube roll power tubes. Anyone know why that would be?"

I've seen quite a few people say that tube swapping shouldn't make a difference in a high-feedback amplifier.  The theory is that high levels of feedback will pretty much swamp any small sonic differences between tube brands.  All I can tell you is that my Williamsons use the standard 20dB of feedback, which is about as much feedback as most vintage push-pull amplifiers employ, but the differences in output tubes are clearly audible to me.  Moreover, feedback cannot compensate for poorly-made tubes that produce below-rated wattage, higher distortion, microphonics and other undesirable artifacts.