Weak link

Sorry I  didn't list it, but I have a single REL S510 subwoofer.

Room treatments.  Hands down this will be the best bang for the buck.  Why spend money on better/more equipment when it will be conflicting with sub-optimal room acoustics????  Good luck and cheers 

If this list of equiptment doesn't sound fantastic then there's something wrong with either the room or the placement/location/toe in/ triangle size of the speakers and proximity to the listening position. You really don't need any more or better electronic gear. IMO.

Is this a dedicated listening room or a family/living room?  Your options may be limited in a shared space.  I highly recommend ASC tube traps as the most versatile tool for shaping and tuning the space.  The downside is aesthetics for some.

Without spending money, I would revisit your setup.  Speaker position and toe in changes of a few fractions of an inch can make a big difference.  Seating distance is another thing to consider.  Inspect cable routing, keep speaker cables away from power cables.

You mentioned window treatments but didn’t state what type.  I like horizontal blinds and wooden shutters.  Varying the angle has a noticeable difference in the reverberant sound field.  
 

Count me as another that who will emphasize room treatment.

Adding room treatment to my current system, delayed other upgrades for a long time, since it improved my existing system.