Suggested upgrade paths

+1 on the Totems.   I like mine!

I have a pair of Super Lintons driven my Marantz KI Ruby CD/Integrated.  In my system I do not find these shrill or edgy at all.  I find them to be a great speaker that excels in real world systems.  They can be placed a foot or two off the front wall and have deep extended bass without any boom.  They image beautifully. And while the tweeter is revealing, it in no way crosses the line where I feel the need to reduce the volume. Quite the contrary.  Different rooms, different ears, different systems, YMMV.

oh, and assume you’ve played with toe-in (e.g., less and no toe-in)?

I don't think that a sub will help the problem you describe. I am a Harbeth fan at this price point for the quality and balance of the timbers. Owned monitor 30.2 with Ton Trager stands. It was an outstanding combo. The 30.2 is known for its mid-range and voices and the HL5+ are not as good in that range but have better bass and are more forward.it depends on musical presentation taste. You have to look at replacing the Rotel 1592 down the road however IMHO. Good luck and keep us posted.

A couple of easy things you can do to address the situation - "the treble is shrill to my ears and the bass not as clear/punchy as I prefer".

1.  Move the speakers farther apart and toe them out a little.  This can reduce the treble shrill, and has worked for me. 

2.  Check your speaker cables.   I have found that low gauge copper speaker cables improve the bass, thus, reducing the emphasis on Treble...  Try "WORLDS BEST CABLES  - 7 AWG - Ultra-Pure OFC" on amazon.  You can very easily return them for your money back if they do not help your situation.  I got a lot more bass out of my speakers with these exact cables.