The ol' output tube dilemma

Folks,
I've been listening to my VTA-modded ST-70 for well over a year now, using 6p3s-e output tubes. I'm thinking of trying a different output tube; maybe GL KT66? Overall, I'm happy with the sound but I often feel like I could use a little more bass... I'm listening to Klipsch KLF-30 speakers: a dual 12" woofer, horn midrange and tweeter speaker with 102dB efficiency. I've got the "low gain" board, with a center Tungsram E80cc and Sylvania 12BH7.
Now I'm not one to go crazy with bass, in fact if I turn these up fairly loud they're pretty even. I don't often listen like this, however.
I'm hoping to get some opinions of anyone's experience with differing power tubes in their ST-70...how did it change the sound? Which did you like better? What are your speakers?
Any and all opinions welcomed!

I ordered my vta st70 with Sov Tek kt88 tubes. Later, I took Bob's advice and switched to the GL KT 66 tubes, which were better. Then formum member Junky Jan steered me toward the GL KT 88s, and I have to tell you they are much better again than the GL KT 66.

Speakers are single full-range driver double-mouth folded horns. (Now there's a double-mouthful!)

If you have a ST70 that is stock/original then you may want to stay away from the KT88s.. You may burn up your power tranny. Not up to spec to run the KT88s. If you have the ST70 from Bob/Tubes4hifi then you are good to go running KT88s.

I am not sure a different tube would offer much of a difference in the bass depth you would experience. The most change would come from the size of the room and the speakers. My ST-70 has the 6P-3se tubes with mini_monitor speakers plus a solid-state powered subwoofer. This setup is in a small/tiny room in which the deep bass just cannot exist.
If I am remembering correctly, for truly deep bass you must be seated some 30ft from the speakers.
I have used a number of different output(power) tubes and while I observe different tonal balances and quality of sound, I have not yet found a better balanced tube than the 6P-3se. I have not tried the KT-66 or KT-77.
At any rate, this is a good subject for discussion.