What can I do to improve my 2nd system?

My 2nd stereo system is in my 14 ft x 18 ft bed room. The system is placed on the short side as shown. The sound path is laptop/CD/SACD player -> DAC -> power amp -> speakers/subwoofer.  All the details including the placements and cabling are shown in the picture.  The subwoofer is connected from DAC to sub. line-in and line out to power amp thru a build-in HPF fixed at 80Hz 12dB/Octave, which I found the best way for integration bet. the main and sub. The bookshelfs are slightly toed in.  After adding the sub., I like the almost full-range sound out of this system but wonder what else I could do to further improve it.  Any advise is appreciated.