I used several amps with my RM40's, with various results, they are as follows, in order of preference(your mileage may vary..)
1. Antique Sound Labs "Hurricane" - incredible, in ALL parameters. Quick, liquid, phenomenal dynamics, imaging, etc., also tends to let some poorer recordings lose some of their harshness & "bite" do to the smoothness of reproduction.
2. Llanno "Trinity" 400s Very good, incredible bass, not quite as "muscial" as the Hurricanes.
3. Spectron Musician II -- Could never get this amp to sound relaxed and musical. Great detail, imaging, but again, too sterile and edgy.
All three were used with my Audio Aero Capitole 24/192 run "direct", with Analysis Plus Crystal Oval. Had PLENTY of gain with all three.
Bottom line, IMO, tubes with the RM40 are a "match made in heaven".
You should at least audition a big tube amp/s before making your final decision.
Best of luck!