why does triod mode sound better than ultralinear?

The "ultra-linear" configuration was developed by Mullard to sell their EL34 pentode. It generally differs from a typical SET in that the SET usually has two tubes per channel - driver (eg 6SN7, 6922) and output (eg 2A3, 45, 300b). An ul amp will usually have four tubes per channel - an input pentode (EF86), a triode phase splitter (ECC83) and two output EL34 tubes - and it splits the signal. This is a push-pull type confuration whereas the SET is a straight-thru amplifier. The difference is that the SET sees fewer signal manipulation than the ul, similar to Class A transistor vs Class A/B push-pull ss amps. The "ultra-linear" name does not describe signal linearity but instead describes the way the EL34's are connected to the transformer (both the anode and the screen grid are connected - to stabilize the whole setup).

The trade off is power for the signal linearity of the SET (I agree with Marakanetz, SET signal path is usually shorter). Of course, it does not mean one is better than the other - there are excellent ultra-linear designs (Mullard 5-20) that can give the best of both worlds.

Btrvalik -- as you are driving Tablettes, I wouldn't change to triode if I were you. Tablettes are not very sensitive (as you note) &, if I remember correctly, they're not the easiest of loads...

I posed a similar question to the folks at Rogue Audio concerning the set up of my Rogue Audio Magnum 88 amp. Here is the response I received, "with speakers that tend toward the bright side, the triode mode may sound smoother and more natural.For speakers that tend to be laid back, UL may give it a bit more dynamics and or punch. It depends on the characteristics of the speaker".

Gs5556: That was a very good basic description, and answered my question. Thank you.

As as someone new to tubes, you made an excellent choice. I'm on my 4th c-j piece (Prem. 16LS II) and have found their stuff is reliable, well engineered, and made by a company with superb customer back-up.

My experience with Tab Sigs is that they can tend toward the bright and lean. I think it'd be worth trying the triode version (you'd still be push-pull in operation) which most likely would be cleaner but more natural in sound. I had a Mesa Barron which was switchable and found triode to be the most musical with my ProAc 2.5s. I now use SETs but did borrow a c-j 60 (triode version) about 1 1/2 yrs ago out of couriosity. It was wonderfully musical and if I ever leave SET land (fat chance) I'll buy one of them or whatever c-j has current.