Seems to me it maybe reduces power amp drive, which IMO is a bad thing. In any case I don't think it's wise to use an AY or AT or such there. I wouldn't change from original specs in other than V1, where it's quite handy to use a lower gain tube if you want to lessen preamp drive.
I also have a plug-in SS thingy for the inverter. There's less sag and full chords retain clarity better with the solid state 'tube' though.
All in all I don't find there is very much difference with tubes used except for V1, or in a multichannel amp, each 1st tube the signal sees...I rather go for filling the other slots with sturdy mil-spec sovteks for instance.
Just my 2c.
Dee
"When life's a biatch, be a horny dog"
Amps: Marshall JVM 410H w/ Plexi Cap mod, Choke Mod & Negative Feedback Removal mod, 4x12", Behringer GMX110, Amplitube 3/StealthPedal
Half a dozen custom built/bastardized guitars all with EMG's, mostly 85's, Ibanez Artwood acoustic & Yamaha SGR bass, Epiphone Prophecy SG, Vox Wah, Pitchblack tuner plus assorted pedals, rack gear etc. for home studio use.