Really beautiful description nvp.
I would like contribute, but my Accuphase experience is way less deep and interesting
Anyway, I try to add my 2 cts.
I'd say that a really big improvement was the switch from using the 303x as integrated, to using his final section with the C-275 preamplifier (lower noise floor, much more definition and scene separation in all the music spectrum, even if the "honey" presentation of the 303x pre section could be nice and cozy), to acquire the P-700 (total control of the 801 S3: the 303x power section at high levels becomes a bit harsh with those speakers).
The DG-58 substituted the miniDSP 22D, and honestly it is better in some aspects (relaxed presentation, perfectly pitch black noise floor, easy to use, stand alone without computer to calibrate, many more functions) but a little worse in others (calibration effectiveness regarding some room problems). In my case I had to improve the bass passive room treatment to reach the result I had with the Dirac correction alone. Now is fine.
Just for fun, I once switched the 801 S3 with the Klipsch Heresy III I have in second system, and they interact more with the room: with the miniDSP results were good, with the DG-58 much less so, because I think that the Accuphase correction is less capable to eliminate this "interaction" with the room from the less braced Heresy cabinet.
The C-2420 was included in the mix just to have a good and user friendly volume control. Before, I connected directly the DG-58 to power amp. The result is that I don't ear any difference regarding music presentation in my system (I also did some blind tests and measured with REC), wich for me is good news since in my view a pre should not add nor subtract anything, especially if a DG is in the mix (but it's only my view) and I have a source/dac with some "character" (see below).
The DP-75 substituted the DP-510 in main system. I am now using it as a DAC, after DG-58, and I find his R2R presentation more graceful than Delta/Sigma DP-510 and DG-58, wich in comparison a find a bit too much analytic and "lifeless". But I must stress that differences are small and I surely need to blind test it. It could be possible that I am influenced by the soft spot I have for minimalistic mid '90 Accuphase styling
I must add that while I always had the same speakers during my small journey, so a variable less and good for comparison purposes, maybe now they start to act as the bottle neck and don't let me fully appreciate present and future improvement. We wiil see