Obviously the room, although it needs slightly unique solutions for open baffle speakers. I ran various models of open baffle Alon's for years, and I previously owned PL I can't recall if it was you or someone else, in any case there is recent thread on mods for your PL. Open baffles like to be well away from room boundaries, but then you may lose bass moving them out further into room. You could also try EL34 tubes with PL, I always preferred EL34 to KT family in push pulls, EL34 has magic mids the others can't match.
So, room becomes somewhat moot if audio specific treatments not allowed, just get the speakers well away from room boundaries. Clean power is where I'd go next with your system, don't cheap out here, dedicated lines and good grounds go a long way, assuming you can modify your electrical service. Power conditioner would also be at top of list for improvement, you'll get endless advice as to which one. Beyond that your dac is next weakest link, and r2r dac's not necessarily rolled on top, I have r2r and Sabre chip dac, implementation of any protocol or chip is key. Your system very nice foundation for excellent sound quality, don't need wholesale changes.