I have no doubts that HRS frame would be a worthy addition to your M3X platforms. Both frame and M3X are probably designed to work in tandem for ultimate performance.
Few years back when I was searching for components racks, I stumbled upon Core Audio Designs. One look at their design and exquisite finish prompted me to dig deeper. BTW, I currently own three PlyKraft’s and a pair of isolation platforms for my monoblocks.
To answer your question, not all Ply is created equal. The "plyKraft" shelf is composed of 32 layers of Baltic Birch hardwood. I believe the binding ingredient is Phenolic resin. Since I have owned both hardwood (Salamander) and PlyKraft racks, there is simply no comparison between the two. Apple Ply is amazingly stable, incredibly durable and extremely dense/hard.
To prevent ringing, the inner core is lined with butyl resin, aluminum, and high density closed cell foam. Liberal use of clay and lead are used for mass loading and the latter for its shielding properties. The uprights (legs) are solid 8/4 hardwood stock and stainless steel footers.
I am extremely happy with the results, never felt the need to further isolate my components as they sound and look amazing on PlyKraft’s.
And above all, stellar one on one customer service from Mr. Marr.
As ’chayro’ elegantly quoted, if you seek the ’icing’ for your rig, then you should find a way to audition PlyKraft before you shell out $7K for HRS, IMO :-)
Good luck!