Suggested upgrade paths

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.  

Hi , not much room info. I had similar problems but I mitigated most of mine with tube rolling and cables. However in regards to YOUR room, is there a TV in the middle, carpet or bare floor, cloth furniture or drapes ? When you do a simple hand clap , how is the sound ? I was able to again mitigate some of my issues due to carpet and adding cloth drapes and a large cloth sectional. Gik sells some pretty attractive panels including sculptured and the ability to print your photos. Recently I added Townsend Isolation platforms for a HUGE improvement. They definitely won’t pass the wife test , but maybe some less expensive/ intrusive isolation like Herbies. Cheers , Mike B. 

With my Harbeth Super HL5+ XD, the singer is close to live. Voice clear. Mid range I do not see an issue. Of course it's system dependent but they are capable of putting out a full range. Having a well produced recordings helps to evaluate. Helps a lot. 

Changing out speaker cables makes sense. I doubt it will help but it does not hurt to try. Speaker cables do matter.

Please stay away from changing cables to correct this problem, too subtle and subjective. 

The most cost-effective way, as stated above, is speaker placement. Try toeing in, or out