Pentode Power Tubes - Ultralinear vs Triode

op

answer to your question is totally dependent on speaker and its load to the amp

triode mode reduces output 3 db which is substantial on some lower sensitivity speakers and also changes the power tubes' 'leverage' into driving the speaker

so as is the case in many of these questions, you can really only try for yourself and see

DPHP user here.

Check out the KT77 is you like the EL34 sound. To my ears they are a little more dynamic overall. Look at KT88 for more on the bottom end at the expense of less midrange magic.

 I have been using the old RCA 6l6G coke bottle in my PL. They are a lower power(different from modern 6L6). They are a close enough combination of EL34 mid and KT88 dynamics.
One of the few NOS American power tubes still available for a reasonable cost.
https://www.ebay.com/itm/RCA-6L6G-Smoked-Glass-Tube-NOS-foil-twin-D-getter-copper-grid-posts-code-35...
They have a "dry"sound which to me is more convincing compared to the "lush" EL34 tone.

I prefer the triode setting most listening sessions. A flat recording sometimes sounds better in UL. You are aware of the 2 innermost preamp gain tube also will fine tune the sound? Subtle, but another tweaky option.

Be an early adopter and try the KT170
https://primalunablog.com/2020/10/30/tung-sol-kt170/
They should be like a supercharged KT150, which already is a powerful tube. I haven't tried them in my amp yet. 8=$$$$

I use a non PL amp triode wired pushpull in class A running a quad of KT-77's and it sounds fantastic.

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