Digital Room Correction: Where in the chain?

 Between your preamp and amp, I believe that’s what’s recommended by the MEN owner’s manual for your type of set up. Anywhere else will bypass some of the inputs and negate the whole endeavor for those bypassed inputs. From my listening demo of the unit the benefits far outweigh the down side of the ADC-DAC.

Yes, that’s the ‘only’ way the MEN220 works. And I it does seem most users are quite happy with the results. To rephrase my question - ideally, wouldn’t it be best to do any of that DSP upstream and eliminate the ADA? Is there a good alternative to the MEN220 approach?

The way the Legacy Audio Wavelet does it is after the outboard DAC or preamp. Then the final signal is sent to the amp. (And speakers if using the XD versions)

At that price I wouldn’t buy it without an in-home demo.  My experience is really good room correction can be more than worth the additional A-D-A conversion, but it can obviously vary depending on system, room, level of treatment, etc.  The other option is to buy used and hopefully sell at minimal loss if it doesn’t work out.  Best of luck. 

https://www.usaudiomart.com/details/650241532-mcintosh-men220-room-correction-system-complete/

The problem here is applying the correction to lp. It would be better to apply the correction to a digital signal, before the dac.  I assume most audiophiles resist disturbing the all-analogue chain of lp’s and tape (otherwise, what’s the point?) But if you want to apply digital signal correction to lp’s, you’re stuck with a conversion.  Putting the correction between the pre and amp at least avoids a double conversion for lp’s, but it will apply the double conversion to the digital signal. 
  Can’ you use the MEN (or something else, like Accuphase) as the sole dac? Does it have digital inputs and a volume control?  
 It challenges my deep audiophile prejudices to have 2 dacs in the chain, but it’s a trade-off.