Pentode Power Tubes - Ultralinear vs Triode

Thanks for chiming in. Like so many other things, it seems the devil's in the details. Someday I'll know better than to think I can come up with a simple rule of thumb without more exceptions than rule.
I suppose when you start getting into tube rolling and mode of operation the changes are subtle enough that everything else has as much or more influence. I'm surprised no one has mentioned changing modes with different cables yet.

KT77 were already next on my list of tubes to try. I got the 7581A because a lot of their described attributes were also appealing to me, and they are about $100/set cheaper than the KT77. For my speakers and listening habits I definitely don't need more power and I don't feel I'm missing any punch. I keep trying things and muddling through because "You can't get what you want, till you know what you want."

Triodes are more linear and have a lower output impedance. If you can get them to make the same power and if you have two amps, one optimized for tetrodes or pentodes, and the other for triodes, the triode amp will sound better hands down. This is assuming that the build quality and topologies are otherwise identical.

'Better' in this case means lower distortion, so more neutral (since the ear interprets all forms of distortion as tonality), bandwidth will be the same, and output impedance will be lower, so again more neutral with more speakers.

My Dennis Had Firebottle HO (it's a HO...says right there...) is a single ended pentode amp at about 12 to 17 watts (HO is "High Output") depending on tubes. Although Atmasphere's frequent comments regarding SET (Or SEP in my case) distortion make sense, after using various tube and SS amps over decades, my Firebottle never sounds harsh or distorted or anything besides beautiful through high efficiency speakers with all sorts of music at any level I can tolerate. That said, Gold Lion reissue KT77s seem to stick around in this amp longer than KT120s, 150s, 7581As, KT88s (GL or Sovtek), or anything else now living in my crowded tube drawer. 

I only have two amps that are switchable AES Six-pacs and a Prima Luna. I prefer both in ultralinear mode. I have Gold Lyons KT-88s in the Prima Luna and it sounds great with better extension on the top end.  

FWIW if ultra-linear is set up correctly, the result should be a pentode or tetrode power tube operating with the same linearity as a triode but with about 90% of the power it could make as a pentode or tetrode.