Tell you what, when I bought the AF2 (10 years ago) it replaced the rest of my gear and the heart of it was a custom made (my specs) Metro Super Lead (early '69 specs mainly) with several modifications to be able to swap power tubes easily. In my case I was using ancient Mullard EL34's, Winged C EL34's, old RFTs EL34's, JJE34Ls (never bonded with those), a quad of contemporary 6550 Tungsols classified as "late gain" and many more. I was swapping power tubes all the time as it was easy to bias them etc (very large external resistance range 10-turn bias pot etc).
The differences were instantly obvious at all volumes and under any condition. And back then the TS 6550s were my favorites, again at all volumes and I don't use distortions.
As much as I loved my AF2 (spent 9 whole years with it) and now my AF3, swapping tubes on these devices was never satisfactory for me as I had a real world experience with tube swapping on my Super Lead.
I would much prefer if the transconductance was put in the sound/behaviour equation of yours at least maybe the results would be more realistic. But, hey, what do I know.
PS: with EL34s the differences were even more obvious when the volume was wide open, it was very easy to observe the differences in sound, behaviour, compression, how "controlled" the amp remains or not, how the bass frequencies were behaving or if they were all over the place etc. Going from any EL34 quad to a 6550 was night and day accross all volumes, frequently-wise, gain-wise etc. That's my experience.
And I quote myself...remember that Metroamp of mine? I had the chance to play with it again (with a full stack loaded with '69 30W H55s) but this time loaded with a quad of NOS Marconi BVA KT66s...three times in 10 days, several hours each time, volume anywhere from 1 to 10...
And once again, the difference was night and day (compared with the 6550's and any EL34's I've tried it out on this amp), the 6550s used to be my favorites for this amp, well...scratch that...the way they those KT66s handle the bass freqs, great mids and pleasant highs...nirvana...
But I digress, I think I made my point several posts ago...