For the Isis, I think CAT JL3 and BAT Rex Power are your main choices. I don't know of anyone using Audio Research, so they may work, but are with some risk. I considered VAC, but worried about the bass with the 300, and the 450 is out of reach for me. Avalon shows with VTL (not that means a single thing), and the VTL seems to have its advocates and critics.
My approach was to create a sound that transcended what people regard as solid state or tubes. Unfortunately, many people do not seem to think this can be achieved. The most instrumental pieces after a tubed preamp with overkill on power reserves, was the Tripoint Troy and Jade Audio cables. If you want to have a listen, let me know. The sound is so palpable, clear, dynamic, and extended. No noise, no hardness.
My approach was to create a sound that transcended what people regard as solid state or tubes. Unfortunately, many people do not seem to think this can be achieved. The most instrumental pieces after a tubed preamp with overkill on power reserves, was the Tripoint Troy and Jade Audio cables. If you want to have a listen, let me know. The sound is so palpable, clear, dynamic, and extended. No noise, no hardness.