I'm working (rather slowly) on clone of the MPU-401AT card. I own just one original, but need at least another one for next project, but now it's highly overpriced and even more rare.
So my intention is to have an "MPU-401" card with intelligent mode (necessary), wavetable header (necessary) and overal design close to the original MPU-401AT (retain the same connectors - stereo jack, 2x RCA, 2x miniDIN).
HardMPU seems to be good starting point, it has bus interface logic that seems to be very close to the original (at least to what can be analyzed from original MPU-401 schematics), MCU used is good choice and the code is sophisticated enough, may be expanded in the future...
There is HardMPU-WT variant with wavetable header, but there are some things I don't like there.

So I can just take HardMPU, add interface I want and signal circuitry from original MPU and design the layout like the MPU-401AT, easy...

However, there are a few things to think about when looking at the original MPU-401AT schematics.
- Power supply for WT and signal chain
- Signal amplifier

Power supply for WT and signal chain:
MPU401-AT use +/-10V (instead of +/-12V in most of post-designs) for OpAmps power supply and for wavetable module (header). The reason is likely to have clean power suply compared to "rough" +/-12V from ISA bus.
Is it enough for OpAmps on MPU-401AT? Yes. Is it enough for SCB-55? For sure, it is designed for it. It is enough for any other wavetable module connected? I hope so.
Looking at picture of SCB-55 wavetable module it seems that there are two linear stabilizers (IC12/IC13), are they for +/-5V? Tantalum caps behind them are rated at six volts, so most probably yes. DB50XG wavetable module has the same layout, two linear stabilizers(?) and tantalum caps with the same rating. So if they are making +/-5V then +/-10V at the input is more than enough.

Next, what are the requirements for supply current? Looking at SCB-55 and SCC-1 datasheets, there is 18-20mA@+12V and 11-15mA@-12V. I don't know how about other wavetable modules, but if I assume only some link-level amplification on board, lower tens of mA's will do. Together with the current supply for OpAmps at the MPU401-AT board the total current at both analog supply voltages shall not exceed 50mA. Let' say 100mA designed current capacity should be enough. 10V instead of 12V for WT module also lower power loss (heat) at its linear stabilizers a little bit.

So what use to make +/-10V at MPU-401AT clone? Discrete linear stabilizers? Preferably LDO type as there is no big margin in input voltage. Any tips and ideas?

Signal amplifier:
There are NJM2060 OpAmps used on MPU-401AT, two of them for each channel. First one has gain of 2,75 and the second is buffer with gain of 1. NJM2060 is obsolete now, any tips for a "modern" audio OpAmp replacement?

There are some notes about "too loud" WT boards out there, I don't have any of these, consider it in the design? (add variable gain to signal chain? What numbers are we talking about?).

There is a power-on-mute circuit in line and phone output, a handy feature I would like to keep in the new design. Roland also used a lot of EMI filtering in analog and power lines, often omitted in new designs, useful for old hardware with half-dried capacitors and lot of noise.