Best speaker for BC2 amps.....

I would second the Merlins (VSM se or M).

Ditto for me. Have owned three different generations of the Merlin VSM (Gen 3, SE, and Millennium) and am very happy with it. I understand Bobby of Merlin used Blue Circle amps to demo his speakers in past Shows before going with Joule Electra OTLs.

Also consider Spendor and Harbeth. I use the Spendor SP 1/2 in my BC based system, and the combo is magic. As for the Martin Logans, Gilbert uses them in his reference system. In fact, I think there's a link to his system on the BC website.

I'd vote with the Spendor and Harbeth crowd. When I heard BC 2.1's with Merlins, they were very assertive. If you like that, fine. Nothing wrong with it. But if you prefer a less pushy sound, try Spendor SP 1/2's or Harbeth Compact 7's. Spendors a tad more sensuous, Harbeths have much better bass. Both among the best. Dark horse, with more personality than the BBC speakers: Reynaud Offrandes. More expensive but probably a very fine fit.