Amplifier Power tubes - important to sound?

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.

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.

I thought that was odd too. Maybe a weird typo?

I’ve used a Williamson amplifier (pair of Heathkit W5) with Gold Lion (Russia) KT66 and it was very "powerful" sounding with strong bass, good "slam", and plump full mids. The same amp, fitted with vintage GE 7581A (near KT66), sounded more airy, spacious and articulate with a gorgeous midrange - but leaner in bass.

Sadly, power tube rolling becomes quite painful in amps using more than a quad. Vintage/NOS is pretty much out of the question. BUT once you get so many push-pull pairs in parallel - that’s where the real magic and beauty of tube power lies for me. On just one push-pull pair per side, I will certainly prefer the tonality of EL34, 6L6GC / KT66 over KT88 - at least until I run out of power,