Might also check that some particular filter or auto EQ setting didn't get activated.

Your description is understandable vague but it sounds a bit more 'real problem' suspect. However, I once thought I had a real problem for a few days until I finally discovered that my receiver had somehow been set to "Flat" auto EQ setting instead of "off" where I liked it.

And lets not bring up the cat toy in the sub woofer port incident.....

With great power comes Awesome irresponsibility.