I have heard the C-50 and it is fast and musical as you have read.
Please message me for more details.
I've owned the GNSC Reference modded RA Opus 21 and s30 for several years. I recently purchased the RA Cantata Music center. I've heard and demoed the C-50 extensively. I did not purchase the C-50 because it did not have the second set of speaker connections that the s30 had.
I would say that the C-50 is slightly on the warm side of neutrality. It is definitely not a overly warm/lush sounding amplifier. It is very open and and fast and very musical. I heard it drive a pair of loudspeakers that were 85 db efficient with no problem. However, I wouldn't think that 50 watts with your loudspeakers is going to blow the roof off your house :) If possible, I would try and demo if you can. Good luck.
I have an Opus 21 stack, consisting of the Opus 21 CD player, power supply, s30 integrated and XS ("Extra Sources"). Mine, too, was modified by Great Northern Sound. I understand that the C-50 is based on the s30, which had lots of good low bass, provided that you don't mind living within the power limitations of a 30 watt amp. I listened with my Verity Parsifal Ovations, and they sounded fine at moderate volumes; I don't listen too loud under any circumstances. I use my s30 now with Spendor LS3/5a's in a very small room and find that highly satisfactory, all of the volume limitations above restated here for this outfit.
I understand that the C-50 has a lot the same sound as the s30 but has a bit better resolution and twice the power. Since Steve Huntley is representing Resolution Audio now and has a ton of experience with both the s30 and C-50, why don't you contact him at Great Northern Sound Company (651.351.5355) to get his input. He is not a dealer anymore and would refer you to a dealer near you if you want to purchase a C-50 but he is a straight shooter and his opinion is likely to help you.