The Acoustat direct drive amps are transformerless with solid state input and tube output. They put 5000V on the panels. I use them on my Acoustat Model 2 in place of the magnetic (transformer) interface.
I have Zero’s between my Atma-Sphere’s and Quad 57s and have a Music Reference OTL with built in autoformer (although the soon to be released MR OTL won’t have one). No ill effects using either. Actually building an autoformer volume control as we speak to replace my Beveridge direct coupled preamp so I’ll have plenty more wire in the signal path.
I have Zero’s between my Atma-Sphere’s and Quad 57s and have a Music Reference OTL with built in autoformer (although the soon to be released MR OTL won’t have one). No ill effects using either. Actually building an autoformer volume control as we speak to replace my Beveridge direct coupled preamp so I’ll have plenty more wire in the signal path.