Go somewhere to audition at least your speakers as setup with the 686 in center then swap for an HTM62. A proper center may make the difference.
Also, where are you in relation to the center and what's its position (vertical/horizontal)?
As for the amp, how did you calibrate it?
As a matter of preference, after calibration, I bumped the center 4dB.
Onkyo 5008 with CM5s in front and a CM Center and it's fine. I calibrated with Audyssey but mine uses XT32. Older versions don't sound all that great IMHO and may be your issue.