Can Room Correction Be Added to an Existing DAC?

The original stacked quads were in a stand made by Mark Levinson. There was a Decca ribbon tweeter between the two and the system used 30" Hartley woofers. It used 6 ML 20 watt class A amplifiers. I think they were John Curl designs. The Preamp certainly was.
The Quad 63 does not have side plates. The best way to do this would be to turn the second speaker upside down and place it on top. You will have to fashion a bracket between the two and hopefully the whole mess will balance on #1's stand. You will need a cross over with a high pass filter and I would recommend crossing over at 120 Hz. You will need at least two subwoofers. I use 4. With a full function cross over you can use passive subs with an outboard amp. Fun! The Quads start rolling off at 
10K. They are 5 dB down by 20 K. You can fix this with room control.

Wow! Thanks. Actually I got the 63's with Gradient basses, stands and crossover. So maybe doable. Just need a little space :-)

If you want a top of the line solution you can check out Bacch SP. They have a version with built-in dac and one without. The most expensive version also has head tracking!

https://www.theoretica.us/bacch-sp.html

My dealer is experienced with Anthem and set up the 2 AVRs in my house.  I’ll query him about Dirac vs the Paradigm .  I am also wondering that I should go this route, should I add a sub to the two channel system, because the biggest improvements that I hear with ARC are bass oriented.  OTOH, my speakers are B &W 803D, and they are pretty full range

The MiniDSP SHD Studio is a much more modern component, with much better quality internals than other MiniDSP products. I would say say hardware wise the Paradigm PW Link is in a similar class as the MiniDSP 2x4HD. The SHD Studio is in a different league altogether.

Dirac and ARC are generally considered to be similarly good, with many people giving a slight nod to Dirac. I believe Dirac is much more flexible, allowing you to tweak the target curve to suit your needs exactly.

minuDSP does include Bass management upto 2 subs which is very flexible, but the Bass management part has a steep learning curve.
The benefit of a sub with good speakers is not so much better/deeper bass output, but more of flexible positioning to reduce room modes. I would recommend getting a UMIK-1 mic and downloading REW (a free audio analysis software) to measure your frequency response first, before deciding if a sub is needed.

Most people/rooms benefit from multiple bass sources, so room modes cancel each other. However, In my case my speakers when measured individually are quite flat, However when I measure the combined L+R response with a mono signal, I have all kinds of peaks and nulls in the bass region. Adding a sing sub, introduced some crossover anomaly but restored a flat bass response. With a little tweaking of the crossover, my overall combined response is now fairly flat.