I have the ST100 paired with the RP7 (a bit out of your budget), but not surprising they are an excellent match. The RP7 with its stock JJ 12AU7 tubes is "neutral" as least as far as tube setups go, but easily bests my previous Halo Int for treble extension and mid range smoothness. I've recently replaced the JJs with Radiotechnique 12AU7s and couldn't be happier, as they are a big improvement over the JJs for both bass and treble extension, with a dose of warmth. Yes, it now sounds more tubey. : ) Perhaps a used RP5, with future tube rolling is an option for you?
The only other preamp of sorts I tried with the ST100 was my ADI-2 DAC driving the ST100 directly, and it sounded surprisingly good, though not at the level of the RP7.
Along those lines, I have Amperex Bugle Boy type 12AX7 coming in soon, and will try those out in the ST100. It's so much fun...
Along those lines, I have Amperex Bugle Boy type 12AX7 coming in soon, and will try those out in the ST100. It's so much fun...