@traudio
The 7' ceiling you have means your tweeters are about 42", or exactly halfway up your walls. Sound waves will bounce off your ceiling almost immediately. While you have acoustic ceiling tiles, as I do, I can guarantee you are having comb filtering issues.
Those few ceiling tiles can be replaced (where the reflection occurs) with Soundsulate Acoustic Drop Ceiling Tiles, or something like it. Armstrong, or similar types of acoustic tiles sold in Lowes and Home Depot provide moderate reflection of sound but if you measure your vertical dispersion your speaker sound is hitting the floor at the same time it is hitting the ceiling at absolute opposite points. Find the bottom, then look up. About 20 degrees.
If you were to punch in your room size 26 x 13, AI will tell you that you will have a primary standing wave at 21Hz and 42Hz, respectively. Big woofers will excite those specific frequencies, resulting in bloated, muddy bass.
In order to smooth out those frequencies as you hope (and it may be easier to do it with two than one -- so you are on your way!!) to achieve, you must cancel all the room modes, find the right crossover, and perhaps use DSP or room correction, which would really be super helpful to you, considering what you are up against.
You have a lot going on in that room and adding more seems counter-intuitive but it is the journey, or the frustration- or the anger!- that leads to selling and buying new gear.
I had Tower speakers and instead of addressing the ceiling reflection (my ceilings are 7ft 7inches), I replaced them with bookshelf speakers and problem solved. Tower speakers have more vertical interference and send a taller cone of sound toward the ceiling. What's more is that those sounds bounce off walls so you are then tasked with having secondary reflection points that come from the ceiling.
Your equipment is really great. You do have all the pieces, but can you get them to work in harmony?? Hope so! Good luck!!
