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So on further investigation (for anybody who runs across this), you really need some kind of pre-amp to correct the impedance. A transformer/DI ~sort of do it, but it's noisy and not correct. I have a couple of different op-amps in the mail to see if I can come up with a configuration that mimics the stock tone and perhaps allows me to up the gain and lower the rail voltage so I can use a standard 12 or 24V AC wall wart for power. Should make for a fun winter project.
After replacing the filter caps on the power supply, re-routing all the AC lines, getting rid of the AC to the cheek block, redoing the mess of ground connections, and cleaning up all the grounds, the amp is acceptably quiet... Noise is also deceiving on the 140 when you're tinkering since the lid has to be installed to properly shield the pickup. The stock power amp seems to have lots its magic smoke in the process and now makes even less (good) noise but taking the signal at the volume pot (which on this piano is inserted directly between the pre and power amps) provides an excellent quality clean output for the time being and is easily reversible.
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