Hgabert - This is what I wonder: could tetrode mode be inherently superior in some ways, or are your observational results mostly a manifestation of the power issue? I have a feeling that all inherent superiority may actually lie with triode mode, but that for certain music - played back beyond a certain volume, through certain speakers in certain sized rooms - the power advantage of tetrode will out, even with 450's. The comparison in your case would presumably be your 450's in tetrode vs. a pair of 750's in triode. I realize that the 200+ watts available from your amps in triode seems like plenty of power, but I can sometimes reach the limits of those same watts with my amps in tetrode, and I think there's a reason VTL makes the 750's and 1250's.
As mentioned above, I was more of your frame of mind before I swapped the 6550C's for the KT-88EH's. The 6550C's didn't quite seem to thrive in triode the way the KT-88EH's do. And as of very recently, the amps are now powered through an ExactPower EP-15a voltage regulator/waveform correction unit (along with the rest of the system, followed as usual by the balanced power isolation tranformers in my API Power Wedge Ultra for the sources and pre), and there's even more physical specificity and consistent clarity on tap in either mode.
As mentioned above, I was more of your frame of mind before I swapped the 6550C's for the KT-88EH's. The 6550C's didn't quite seem to thrive in triode the way the KT-88EH's do. And as of very recently, the amps are now powered through an ExactPower EP-15a voltage regulator/waveform correction unit (along with the rest of the system, followed as usual by the balanced power isolation tranformers in my API Power Wedge Ultra for the sources and pre), and there's even more physical specificity and consistent clarity on tap in either mode.