I have a FirstWatt F8 amplifier, which has a similar power output rating. I tried an Emotiva PT-1 preamp. Not really bad, but too bright. I swapped in a Van Alstine Transcendence RB 10 tube preamp. Now the sound is excellent.
If you give the INT-25 time to settle in and like the result, you are done. If not, you could even try feeding the output of a tube preamp into an INT-25 line input.