If great music is your goal, I would not touch your receiver, but few other things.
First of all, if your room is really square that is a big issue for any speakers - the bass will always sound muddy no matter how much you invest in speakers/electronics
Imo you should start by doing acoustic room treatments. Start it by visiting a specialist web site online calculator here:
Free Online Room Acoustics Analysis
After that done, consider speakers next:
If you desire to have surround sound you are very highly recommended to have center tune matching to mains and usually the only way to do it is to get center from same brand/series.
In your case FRx-Center which is no longer made since 2001 and will have harder to find on the used market.
You might have to sell/gift/reuse your mains for something else and get whole new 5.1 speakers. The good news is only 3 front must match, the surrounds don't,
In fact you could have your Mirage FRx-Seven towers for surround speakers
There are many good brands which offer in home trials - this is only good way to see if you like their sound reproduction.
At $2.5k you have plenty of options here are some of best options:
Ascend Acoustics
Aperion Audio
EMPTek
Salk Sound
For surround music ideal setup is to get 5 identical speakers. I think Ascend's SE340's fit the bill nicely