Several good choices here and on USAudiomart. There is a Music Reference RM5 mkII & mkIII available on both sites. I had the mkII for 5 years (many years ago), it was a great preamp. Mine was an early RM5 in silver upgraded to a mkII. I loved it. Very neutral sounding, 3 - 6dj8 tubes. Others I would consider are the cj PV5 or Counterpoint SA3. Moscode Minuet in A is also supposed to be an excellent preamp too. I have also heard very good things about the Mapletree preamps as well.
My Music Reference mated up extremely well with everything I put on it (B&K ST140, McIntosh MC225, B&K EX442, Paoli monoblocks, Music Ref RM9) except for the cj MV50, which it sounded out of phase and was looking for a PV5 at that time to mate up with the MV50. In looking for one of these older 80’s tube preamps, its good to note that they have been recapped and upgraded.
An updated Dyna Pas preamp is also a real sleeper as well. I would say the Dyna and cj preamps are probably more tubey sounding than the Music Reference. Perhaps you could try 2 tube preamps - the Music Ref and a PV5 or Mapletree and see which one mates up better with the Adcom 555 amp. You might prefer a more or less tubey sound with the Adcom 555.
