Hi Brian,

I prefer the OA3 /VR75.  This is what ships in the amp.  I prefer it because it makes the most power which translates into headroom.  This also makes it compatible with more speakers which is somewhat the idea behind the TORII... to get the Zen SET sound with a wider range of speakers.

If you want to live with a small drop in power (instead of a big one) and make a big change to the sound along the same lines, get a quad of Tungsol 6L6G and bias the amp up for those.  It's a more romantic sounding tube, a touch darker, less neutral.

Steve