apstoltz: This loooong post is made with more than 40 hours on the HDX1s clock and it goes without saying the following is based on my personal experiences and with my (comparatively) lowly system. And yes, I'm still liking them :) .
For me, the HDX1s have more field depth than the MA4s (pre 2018 design), i.e. the front-to-back depth and, slightly, the height. With the HDX1s, the sound stage seems marginally wider, maybe 1/2 to 2/3 of a speaker's width each side and the overall "picture" seems more open ("air"?); this being fairly close to the Atlas Equators.
Given the differences in materials of the two sets of cables, it's probably not surprising that music sounds a touch warmer with the HDX1s compared to the MA4s but that's a statement of fact rather than a compliment or criticism as "warmth" or "brightness" is pretty much a matter of personal preference. It's only fair to acknowledge Morrow has since changed its cable design to all-copper wire which *may* now make a difference.
With the HDX1s, the bottom end is just a little fuller although, perhaps, not quite as tight as with the MA4s. Mid range may be just a fraction richer but that's getting into the territory of "could I honestly hear a difference in a blind test?" However, for me, there is a distinct difference in the top end with the MA4s occasionally tending to being "toppy" and, overall, the HDX1s providing a "rounder" sound, but without being dull.
At the risk of a much overused and fairly nebulous word, I find the HDX1s to be slightly more "musical" than the (old) MA4s. It can't quite put my finger on it but, for me, there's just a bit more "smile" in the music with the HDX1s (in the listening sense, not the EQ sense).
IMO, the HDX1s (and indeed the Atlas ICs) provide a more enjoyable listen than the MA4s and, interestingly, none of the three sets of Morrows I have had (including the MA4s) have resulted in the earth shattering musical revelation one might expect if relying solely on the sales pitch(es). Curiously, the most impressive of the three sets (IMO) was a pair of lowly MA1s (pre 2018) which did, for a while, displace some (very) old QED and Cambridge Audio ICs.
When Morrow changed the design of its cables in 2018, I was so so tempted to move up to the MA7s (with a massive discount) but, for some reason, my spidey sense went into overdrive, especially following a couple of rather underwhelming replies to product enquiries. In light of my experiences with the Atlas and MG cables, I am relieved my sixth sense intervened and, unsurprisingly, Morrow cables are unlikely to play any further part in my hi-fi adventures.
As for the Atlas Equators, music has a fractionally softer bottom end compared to the HDX1s but, in truth, I'm not sure I'd make that difference in a blind test. In time, I'll doubtless do a direct A/B listen (Atlas vs MG) just out of curiosity but I have no immediate desire to return to the Atlas Equators. It may or may not be of interest that I have a pair of Mogami RCA interconnects (2497 wire) and they really didn't do it for me at all (example: the decay on all of the other cables mentioned was much more sustained and resonant than with the Mogamis). In fairness, it *could* be the Mogami's plugs but I won't be retesting with different terminations.
To close, I found out about Mark Grant cables by chance and happened to be in the mood for a bit of web surfing. There are a couple of old reviews relating to legacy versions of MG's current range(s) which are informative and MG also posted on a forum (artofsound) an explanation why at least three earlier models of RCA cables were replaced with just one (the HDX1). In short, MG considers the HDX1 to be at least as good as the higher specified of the earlier cables, so the "extra" cables were discontinued. If you want a cable fitted with a particular termination that isn't standard (or listed), I have no doubt MG will oblige if he can source the parts.
I hope the above helps more than hinder but, in any case, happy listening!