New speakers or new tubes

@fsgattuso ..."However, maybe due to the lower efficiency of the Thiels, they don’t sound great in triode mode. When the sound mellows out as it does in that mode it is now too flat and lifeless,"

Do you notice it’s quite a bit worse at lower volume levels ...Kinda flat, not engaging, not dynamic and more lifeless? A few reasons for the question.

When you mentioned triode, caught my eye. @soix note too, the comment "to let the 1.5s fully open". Brought a memory.

I read your post and wanted to share something for what it’s worth. For several years I played around building several sets of speakers using the exact same drivers Thiel used from the same parts mfg. Even with quality crossover parts they kinda lacked some presence and body with some amps (not all) and different modes of operation. If I turned up the volume a little it helped some, but not enough with any of my lower watt (10 to 50w) tube amps in triode. Not good at lower volume levels. I too ran subs with my Thiel-like custom speakers. Lower power triode was not a match. Found other ways to make them work a little better - for a while at least, until I discovered different drivers and more efficient / higher impedance speaker designs. I found this though driving these 86db/4ohm'ers:

• Better in Ultralinear mode with my 80wpc tube amps. Tried with 3.
• Different and better with my my other 220wpc SS dual mono amps.

While it might be be intriguing the whole idea of "triode" mode with your PL HP amp, and yes you heard something different with your Tektons, there are some speakers including your Thiel 1.5s that might actually sound better in ultralinear mode for reasons I won’t attempt to try and explain about your speakers. Forget the elusive deep sound stage of triode for a moment. Your speakers need to be lit up some. btw, my new mono tube amps are not triode, full pentode, and they do sound better when listening closely when paired with lower impedance / less efficient speakers more similar to yours.

Question: Did you try UL mode with your PL amp and Theil 1.5s, and truly sit and listen for a good long while, multiple listening sessions? What did you hear?

Maybe worth trying again since you upgraded the 1.5s. Turn your subs off for a bit too, you’ll hear the differences in a different way too between UL/Triode. Then turn them back on, listen again. Hope this helps in some way, fwiw. Best of Luck.

What’s wrong with staying in Ultralinear? You should prefer one mode over the other. I use Triode in my setup, Dynaudio 1.8’s which are also a 2 way + passive radiator small tower. They are probably your polar opposite in terms of voicing. I don’t really worry that it doesn’t sound as good in UL. The differences that you are describing are exactly what you should hear when switching from one mode to the other, and the one that sounds better will depend upon the speaker. Clearly the extra dynamics of UL work well w/ the 1.5’s. I think if you liked both modes equally it wouldn’t be as good of a match. You can try the KT120’s and see what happens but I doubt it will change your preference towards triode.

Actually for not much more than an Octet of KT120’s you can probably find the 1.8 mk2’s. Then just think of that button as the Dynaudio/Thiel switch.

Decooney and perkadin: Thanks you for your posts. Yes the issue is more noticeable at lower volumes. Yes I've done many close listening sessions with both speakers and with sub engaged and not. Triode just isn't good with the Thiels so I am staying in UL. It sounds great, very happy with the Thiel Rythmk combo. Like everyone else, I'm always looking for an improvement so I'm considering standmlunt speakers, Pro Ac Response D2R I think caught my attention. 8 ohm speakers with 90 db efficiency.  

Keep the 34's and get. some speakers that love tube....the 34's can be very very good sounding..