This http://www.untrue.ch/test/ehx_12ay7/ehx12ay7.html is as close as I can find to any type of photo representation of what is inside. You can see the two transistors on the shared heat sink in the top toward the right hand side of the first photo. Plus an IC in the upper left. Who know what EH uses; if they have a house number or a real world number. If you had the right test equipment you could find out what was wrong, or a schematic could let you signal trace it.
If the IC has a real number on it, Google the pin out, and ‘scope the input and output pins if you had a signal generator.
It almost looks like a plain TLO74. Wow. I wouldn’t expect such openness from EHX, but this is a pleasant surprise if it is true. Here http://circuitdiagram-schematic.com/12/pre-amp-equalizer-using-tl074/ is a preamp schematic (NOT the EHX, but just one using the TLO74) to get the pin out and an idea of a typical circuit.
That’s all I know. With real part numbers, you can test anything, and know where else to look. I have had this happen before, and just plain dumb luck helped me. And this was with supposed name brands that had a ‘reputation’. If you have nothing to lose, go for it!

That’s gotta suck. Can you measure any Plate voltage? I realize that may be over most people, but if you can, you might find a missing voltage. The real bummer is that if the voltages seem to be there, you could really be in trouble. Well, if the Plate voltages aren’t there, you’re still in trouble. No schematic, no easy way to trace it out, and who knows what the solid state devices are. Ouch.