System Upgrade

More power will make a huge difference, IMO.  I use a 300 watt amp with the SHL5+.  Flat it is not.  

I agree I have totally different gear. Nonetheless, the most obvious problem that any of us can diagnose on the internet is that his amp is underpowered for his speakers. You need a solid 80 watts for the SHL5+, 150 for larger rooms. As someone else suggested upthread, this can be easily tested without much cost or effort (order a 30 day trial or borrow an amp).

I suppose room placement is another easy and free thing to investigate, but whatever the case, the amp is underpowered. Speculating about warm voicings and such seems to me like medical diagnosis by text message.

I'm gonna go with 

a) manufacturer recommendation
b) peak transient power used in this video with the M40.1s as possible indication of power needs.

https://www.youtube.com/watch?v=bRMR9JZ1m0s

rather than kilobuck cables.  YMMV.

The issue is what we call mud: warm speakers, warm digital, warm cabling, this all creates a sound which isn’t involving and requires you to raise the volume to hear details.

The corollary is sizzle systems: bright speakers, bright amp, bright source, bright cabling = fatiguing sound, over sibillent sound.

He needs to rebalance the system by adding in products which add more detail or sparkle to the sound.

I don't think we have a consensus definition of "mud" (or "magic" for that matter).  As I've said before, I think cables and amps are incredibly expensive tone controls and shouldn't be used as such.  But we just differ on that approach.

I have an off topic but related question I'll pose separately.