why does triod mode sound better than ultralinear?

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.

It is not a question of which is better it is matter of preference. A triode valve has a more linear response than a Pentode valve. This leads to a lower harmonic distortion when the triode is set-up correctly. The ultra linear design is a class A/B amplifier. The configuration of the output valves is set-up to help over come the non-linear response of the valves. It is also worth pointing out it is the harmonic distortion that give a valve amp its "sound". A SS amp has a far lower harmonic distortion figure. The SS introduce mainly odd harmonics because when it is overdriven it clips quickly rather than gradually like a valve amp. A MOSFET amplifier is a good as it gets as far as Hi-Fi is concerned because they are far more linear than any valve however Hi-Fi is not what a valve amps are about.