I've read up on horizontal vs vertical bi-amping modes, but wondered if anyone has tried removing the output tubes from one side of each amp, and just running them as single channel mono blocks using tri-wiring to the speakers? I ran them in this mode for years before taking them out of service for the VTA upgrades. The original logic was that the power transformer wasn't overly robust on the ST70, and using only half the outputs would reduce the load on the PT. As with any mono blocks, there's always some improvement to left-right separation as well.
The speakers are my own design, and were the culmination of many years of making and selling high end speakers as a local business. They're average efficiency, use a SEAS 1" aluminum dome tweeter, Focal 5K013L midbass in an aperiodic chamber, and a Focal 8K412 in a transmission line. Each driver has a separate input jack. Crossovers are passive and proprietary using top grade poly caps, heptalitz inductors, and are wired with Kimber Kable. They're darn transparent IMHO. I don't play them very loud, so they seem plenty content from the 20 watts or so of the ST70 in triode mode.
I'd appreciate your feedback on the pros and cons of these different approaches.