Suggested upgrade paths

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

That's a classic symptom of too live of a room. Whatever you can do to absorb some higher frequencies would help. Carpet, drapes, artistic acoustic panels, etc. are what will help most. 

Not being familiar with your gear, I would start by reading reviews on my gear to see if the issue with sibilance or bass is common to any of my gear.

I would guess that the bass issue is speaker related.  Either placement, the speaker itself, or both. 

Sibilance drove me nuts for years. I tried different cables which kinda helped by rolling off the top end a bit, but it didn't solve my problem.  I just blamed it on poor recordings. Thinking my gear is now so revealing that all of the crap recordings are more noticeable. 

My problem turned out to be a well regarded tube preamp.  Replacing it pretty much fixed the problem.  Upgrading my dac also improved things. Now, I have a revealing system and I don't even think of sibilance. At one point, I had almost concluded that sibilance was just something that I'd have to live with.  I was wrong.