The Acoustat amps used a high-voltage pentode tube, I forget the exact number... I used to own a pair of highly modified Acoustat Servo-Charge amps that used 6LB6 high-voltage transmitter tubes, and they worked really well. As I recall, those amps could drive up to 8 Acoustat panels per channel -- quite a feat!
Roger Sanders of InnerSound has been wanting to come out with a direct-drive tube OTL to power his Eros system, but I don't know if he will ever get that going now that InnerSound has a new partner. But the day he comes out with that setup is the day I'll return to audio reviewing. Hey Rog, you listening!??? :)