This post is just to try to assess the level of difficulty that this sort of build would require.
You can use an external passive control to drive the Smash. Cable in all your sources into the passive box, then run a short pair of cables into the Smash. You can just leave the level control all the way up on the Smash, and there shouldn't be an issues.
I'm also wondering if simply bypassing one of the attenuators would be possible for output to a dedicated headphone amplifier, or if the signal should be attenuated at least somewhat.
The better way to do this would be with a tape output. If your headphone amp has a volume control, then preattenuation shouldn't be necessary.
I have found that, despite the indications otherwise, that
this board can be powered from the 6.3V winding on our PT3/7 power transformers.
The downside to this is that the control board is close to 12" long. You could likely fit it in the Smash vertically with a bit of an angle, but that's about as helpful as I could be in this situation. You can also remove the momentary push-button level control buttons and wire up an incremental rotary encoder in the volume pot hole to maintain some similar functionality to the stock kit. The input selector button could also be replaced with a rotary encoder, though it won't be bidirectional.
I don't know how you would get the LCD display to work in a wood base, that will be tough.
Wiring this up and getting everything to fit will also be tough, and there won't be a ton of support available on our end. Performing the machining and milling to make all of this look acceptable will be even tougher!
The DH-500 specs a 47K input impedance, you could just move your preamp significantly closer to your listening position and run longer interconnect cables.
-PB
