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.