Mr Trev
Headphoneus Supremus
Yup. That's pretty much the impression I've gotten. The 58x is the highest impedance can I have and even that seems to get pretty lackluster results with OTL from what I've read.They do above average (the X2), you'd need/want a 5998 and above for it to shine. The 400i.. forget about it. So far..and I'd hate to publicly admit it....the Beyerdynamic is the cats meow with the Darkvoice. Sennheiser excellent for older recordings. This amp does best with high impedance cans for the low end to shine. The resistance pushes the power up, low impedance cans hardly have any resistance so less power is pushed.
The other amp I've been eyeing is the Hagerman Tuba. It does have a dedicated low impedance output, but probably isn't going to be found for dirt cheap, and I know nothing about EL84 tubes or parafeed circuits.