I had a few upgrades. The biggest has been the DIY bass traps (or broadband absorbers). I fabricated 4 panels of Rockwool. 2 panels of 6" x 2' x 8' and 2 panels of 6" x 2' x 4'.
You can see the 8 foot panels on the 2 front corners.
The 4 foot panels were placed at the rear corners. Here's a shot of one of them.
Of course, after installing these, I had to redo the Audyssey calibration and what you can notice is that the correction curve is flatter than before.
I was really amazed at the difference in sound quality. My bass was already tight but now, it is even tighter. You can notice this most with the kick drum.
These panels are supposed to absorb excess sound energy so you would expect the bass to decrease. Yet, the impression you get is that the bass has increased.
What surprised me the most was that it improved the sound of nearly all instruments, making them more detailed and life-like. I was particularly impressed by the improvement in the piano. Parang live.
This is one of the best investments I ever made sa HT. Parang nag-upgrade ng speakers. Salamat talaga ako kay Alvin, Munskie, Mike_C, Threadlock and all the other guys who gave advice.