I don’t believe output tube rolling is the answer to the OP’s problem, as the below will still be there and explain.
One must remember speaker and the speaker wire introduce not only a resistive load to the amp but a capacitive one as well. ESL’s even more so capacitive, so all amps theoretically should be unconditionally stable, eg no ringing whatever the load on bench square wave tests.
In Ultra linear mode this amp is a little unstable into 10khz 8ohm pure resistive loading, even without any speaker/wire capacitive loading which would exaggerate this ringing even more which may cause the brightness heard.
http://www.stereophile.com/images/214PLDPfig03.jpg But in triode mode it seems to be a bit more stable, and should not sound as bright, but still we don’t get to see it with some capacitive loading just 8ohm pure resistive.
http://www.stereophile.com/images/214PLDPfig04.jpg Even JA "Stereophile) who did the above measurements said this
" PrimaLuna DiaLogue Premium performs at its best with tubes operated in Triode mode."
One can clearly see in ultralinear mode the sharp rising frequency response after 20k this is the rigging seen on the 10khz square waves. "It a bit like an uncontrolled resonance peak of some metal dome tweeters" "brightness???"
http://www.stereophile.com/images/214PLDPfig01.jpg It seems to me that in Ultralinear mode they’ve gone for too much bandwidth (70khz) at the sacrifice of stability.
But in triode mode all is far better behaved, with no peak after 20k.
http://www.stereophile.com/images/214PLDPfig02.jpg If I owned this amp I would only use it in triode mode, and if that didn’t have enough power, I’d get another amp that was.
Cheers George