I also own Vandy 3A Sig's, and have been driving them for about 5 years with a Bryston 4B-ST. The combination has proven very satisfying, and revealing enough that changes in IC's, etc., are quite discernable. I "test drove" a couple of Parasound amps before buying the 4B-ST, and found the Bryston more pleasant to my ear. I've had no regrets about the 4B-ST.
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We sell both and BOTH are outstanding amplifiers! The A21 will actually outperform the Bryston on speakers that are very hard to drive. If you want high current, then go with the A21.
You mentioned the "4-Bst", go with the newest version the SST. In comparison between the A21 and 4B-SST, the Bryston is the better sounding amplifier for music based critical listening. In addition, the Bryston warranty is second to none in the industry.....But, it no longer transfers to a second owner without receipt and registration. And, they strictly enforce the rule!!!
I'm a fan of both Bryston (owned 7B SST for several years)and Parasound, but your comment about Bryston's input impedance is very relevant with your tubed preamp.
The new SST Squared versions have increased the input impedance for some of the amps. The 4B is one - 50K ohms unbalanced, 40K ohms balanced. See page 11 of this doc: http://bryston.com/pdfs/09/Bryston_SST2.pdf
And maybe contact James Tanner to verify.