6.3v is the right one. Unfortunately, my experience with these adapters doesn't quite match bmwr's. The first one I ordered had a horrible buzz in the left channel that increased in loudness as you turned up the volume. The second one, from a different vendor, is arguably "listenable", but has a bit of audible noise in both channels. (Also - from other posters here and on the bh boards, I know I'm not the only one who has dealt with QC issues, so caveat emptor when dealing with the overseas ebay sellers who make these things...)
The only thing my forray into adapters and 6sn7's really accomplished was to show me how very much I love my dead quiet, faultless tung sol 12bh7.
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As an aside, in reference to the discussion of woo amps above - the transformer coupled amps in their lineup are actually faster and more dynamic sounding than their otl options. So you definitely want to stick to the speedball crack, and stay away from the wa3 and wa2 if you found the wa6 and wa22 thick and slow sounding. To my ears, a speedball crack (made even better with the upgrades recommended by beefy above) bests all of these amps by a significant margin, particularly with hd800's.