I've heard the BC21.1 together with a Sonic Frontiers amp (not sure which one) and thought it sounded great. Whether you'd prefer it over your cdp direct to the amp is a personal perference. In my limited experience with cdps w/volume controls direct to amp, I'd say going direct gives you a more transparent sound, but sound a tad flat with less depth and texture than an active preamp.
disclaimer: I am a Blue Circle dealer, and I love their preamps! :-)
disclaimer: I am a Blue Circle dealer, and I love their preamps! :-)