The best ss combo with Avalons that I've heard has been with Spectral. I used to mate Radian HCs with a Spectral DMA180II. The newer 150 would be better. Charles Hansen, of Ayre, founded Avalon, and has been rumoured to voice his amps w/Avalon speakers.
Member Rushton has excellent results driving Eidolons with Atma-Sphere OTL tube amps. He's got the biggies. Not sure how much power & current the Arcus' need, but if the M-60 is enough juice, it could be a sweet combo that wouldn't kill the wallet. With my personal preference for tubes, finances notwithstanding, this might be the best route.
I'd be less concerned with matching to the Capitole. It's the speaker/amp pairing that usually needs higher priority.
Cheers, Spencer