Maxgain, that's very interesting about how the Vandy's amps get their signal. Perhaps the 6L6 wouldn't be the best in this situation. They are cheap to experiment with though. I found the Golden Dragon KT88 to have as much bottom end as a Sovtek 6550. The Sovtek seemed a little compressed. But the Svetlana 6550 is great. Very linear. Better slam factor than the KT88.
Kana's suggestion of the KT90 is a good one too, if you can run them. Make sure you get the type 3, matched and tested. They have some quality/reliability issues, but the ones I got from Upscale Audio were very nice. They took a long, long time to burn in and settle down, perhaps 300 hours or so. But in the end, they offered a solid state like low end, and great linearity. In a Sonic Frontiers SFS-40, they outperformed a pair of bridged Adcom GFA-555's in the low end!
Kana's suggestion of the KT90 is a good one too, if you can run them. Make sure you get the type 3, matched and tested. They have some quality/reliability issues, but the ones I got from Upscale Audio were very nice. They took a long, long time to burn in and settle down, perhaps 300 hours or so. But in the end, they offered a solid state like low end, and great linearity. In a Sonic Frontiers SFS-40, they outperformed a pair of bridged Adcom GFA-555's in the low end!