I was about to respond to the OP but stereo5 took the wind out of my sails. I would not have guessed that the RP-5 would have been a good match for either of the amps mentioned. Insufficient voltage at its rated distortion to drive the MF 2550 and too high an output impedance to match the Odyssey. Just goes to show you personal preference can trump specs. There is no substitute for auditioning with your own equipment if you are trying to match a tube pre-amp to an SS amplifier.
If I were shopping for a pre-amp for a 4B3, I would be looking for something capable of 2V rms at its rated distortion with an output impedance about 200 ohms or less to create my short list, then keep my money in my pocket till I could audition candidates.
If I were shopping for a pre-amp for a 4B3, I would be looking for something capable of 2V rms at its rated distortion with an output impedance about 200 ohms or less to create my short list, then keep my money in my pocket till I could audition candidates.