Room Treatment. Floor to ceiling in the corners, first reflection points. I use GIK, they have very nice art panels. You can DIY it as well. I, like you, made my own rack with 3.5 in butcher block and brass threaded rods.
I will go along with Raul as to the second sub. Order Jim Smith's book "Get Better Sound" , an overall viewpoint on all sorts of tweaks as well as subs, placement, room treatment. As you add the sub and at the same time tame the bass and the reverberation time your system will come into focus and be much more enjoyable. I did mine a little at a time to give me some time to let things settle.
I will go along with Raul as to the second sub. Order Jim Smith's book "Get Better Sound" , an overall viewpoint on all sorts of tweaks as well as subs, placement, room treatment. As you add the sub and at the same time tame the bass and the reverberation time your system will come into focus and be much more enjoyable. I did mine a little at a time to give me some time to let things settle.