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.

Regarding the small tubes, I have an integrated 300B (Elekit with the new Western Electrics) and recently replaced a 5751 (superior substitute for the 12AX7 IMHO) with a Cryotone 5751, the brand  which DecWare offers as an upgrade.  I also added a pair Cryotone 12AU7s.

After playing it for a couple of days of break-in, I noticed I was losing interest in listening to music.  It sounded fine but the emotional engagement was missing.

On a fluke, I switched my previous NOS Sylvania 5751 back in (did not switch the 12AU7s) and Voila!  I couldn't stop listening to my rig!  The emotion, dynamics and tonal accuracy were back!

I had no idea what a seemingly insignificant tube could do.

@roxy54 "It’s so strange. I want to believe that your just putting us on..."

you’re   GOTCHA ! 

Siemens never made a 6550. 

@dogearedaudio : I don’t believe you. The 807 uses a top anode cap and a seven pin socket. It is not swappable with octal power tubes. I had some in my tube collection.