SED el34's are superb...if you like a warm rich midrange. I currently run EH 6CA7 in AES Sixpacs and love them. They sound more accurate in triode than the SED and the highs sparkle more. According to Cary, the V12 was voiced for el34 tubes even though the others would be just fine.
I also have Tungsol reissue 6550's which I like as well --more linear and extended, but that el34 sweet midrange isnt there any more. A sweet looking tube for certain though.
Since the V12 is switchable between triode and UL, which mode do you prefer? The answer to that will help guide your decision...