Opinions Only, so far!
I have had the TAD-150 pre-amp for 4-5 months and love it. Worth 3 times the price IMO. The amps are my second purchadse from Paul, who BTW, is a really knowledgable, personable guy with which to deal.
If I had to describe the difference I would say that IMO triode mode is sweeter and more harmonically rich, better suited to smaller scale music such as chamber, jazz and other simply recorded music. Ultralinear is best suited to large scale orchestral music, rock, opera, and other really dynamic music, Holst: Suite for Bands on Telarc comes to mind. If one genaralized then triode bass is less controlled than UL, which has perceptibly more extension and control at the bottom end. FWIW, I am far from having the measure of the 1000's so these are simply initial reactions. I can say that these amps are a steal at the price.
For a good discussion of the different output topologies of tube amps visit the following link.
<a href=http://ken-gilbert.com/techstuff/vtf.html>Tube FAQ's<a/>