Pentode Power Tubes - Ultralinear vs Triode

i would concur with others that the modern russian kt77 labelled gold lion genelax (gold and red box) is pretty much the best sounding current el34 variant

good linearity and drive with a fair dose of tubey magic 

I have a PL Dialogue integrated. Hands down the KT150 is my favorite and my change of pace the 7581A. I am a Streamer using the Roon interface. I like Ultralinear for Radio and Triode for everything else. I have heard others mention they prefer the EL34-7581A tubes with the PL HP power tube array. It’s hard for me to imagine going to any other power tubes since I seem to have found exactly the sound I’m looking for

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.