I've got a whole thread going here here on this very subject. Lots of good ideas on how to improve the situation, not the least of which, of course, being a switch to the SS.
I actually have that same frustration as you, and had the same concern about my ZLX (that I maybe had blown the tweeter), coincidentally with the new CP4 and older MP6. But acoustic guitar still sounds crisp and clean through it, so I know the drivers are still intact and the problem is some other combination of circumstances. Some cabs just seem to have an issue with AP's that no amount of EQ can sufficiently resolve. I have another older Peavey keyboard amp that was the main sound source for my music room (no handles or outer protection, just black painted wood finish, so I think it was intended for studio use) which I thought sounded good when I first got it for general keys, but the first good stage piano I fed into it sounded like crap. That's what prompted the earlier and very fruitful search for decent studio monitors. It's obviously not a unique problem, and I'm frankly kind of surprised about that. It's looking more and more like I'm going to have to plunk down for a SS. (Yes, dear, I have to.)
Yep, I've been following your thread. Sounds like we're in a similar boat (except you're using these for your whole PA system and covering bass). I'm still on the fence on the SpaceStation. . .thinking of going with a pair of EV ZXA1-90's as they tend to get favorable reviews for AP sounds. I don't need a ton of bottom end as I'm always playing with a bassist and have FOH support. Just need to find something with a flatter response.