Weak link

If room treatment of some sort is possible, I'd start there. 

weak link 1 = lack of subwoofers

Get a Elac Varro DS1000 and use the built-in calibration tool it comes with (very handy tool for all subwoofer noobs). 

weak link 2 = lack of power conditioner

Get a Niagara 3000 or 5000

+1 @knotscott I would spend some effort treating the room.  You have very nice components.  Room treatment is likely the best bang for the buck.  A pair of subs would be interesting as well.

To refrain others, without a budget, photographs, and a room description/dimensions it’s hard to make recommendation on your solid system.  I would address the following, in the order listed, for both will have a positive effect on SQ:

1:  Power quality.  A good conditioner like a Shunyata Denali or Everest, or an Audience Adept Response T series.  Then, if your home design permits without much construction cost, dedicated 20 amp power lines.  If possible, one for digital and one for amplification.  
2:  Room Treatment. 

That’s a great system. +1 for dedicated lines. If you are technically inclined get a usb mic and fiddle with REW to see how your room rates. Then look at room treatment. I have tried and like the WireWorld Eclipse interconnects, but you might try some other ones. I found them a touch on the dark side. I have not tried the ww speaker cables, but worth some research if you are on the lower end of their lines.