Weak link

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.

is there a vaccine for upgradeitis?  I can't imagine your system not sounding fantastic but yeah I understand the disease "What am I missing???"  There is no cure but there is always ways to stave off the impulse to mortgage the kids and provide temporary relief. 

Cleaning all the contacts could be all it needs to get that last ounce of nirvana.

Moving your speakers is also great pastime (don't forget the blue tape to  return them back).  I'd also try moving your sub to different places in the room, closer to the wall or pulled out more into the room,  away / closer from the corner .   As you've become attuned to the way your system sounds that original setup might not be optimal anymore.

If you really need to spend money a couple of acoustic panels leaned up against the wall at reflection points would give you some ideas what treatments could do and if like what they do.

The last option if all else fails is for me would be to add an additional sub (or two)  to address the inevitable dead spots in the room.

It's the journey... remember it's not the destination but the journey