Entertainment Center: TVs, Stereos, VCRs and DVDs - Help! Sony STR-DE197

I have a humble, makeshift sound system composed of a Sony STR-DE197 Receiver, and two old Yamaha 200w speakers I bought from my neighbors garage sale a couple years back. I have been extremely pleased with this setup until recently, when the output stopped sounding good. There is noticeably less bass, and the sound seems a bit muddier in general than in the past. However, it doesn't *seem* like the speakers are 'blown'. There's no crackling or anything, just a kind of muted sound. I first noticed it after watching a movie with the system on a very loud volume, but I would be surprised if that was the cause because I have played music at that high volume on them for years. I have tried using different audio sources, both A and B inputs on the receiver, and swapped out for new speaker wires, none of which made a difference. I don't know if the problem lies with my receiver or with my speakers. Please help, thanks!

At 100w per channel, the amp is underrated for the 200w speakers. In this setup, sustained loud volume can cause the amp to clip (distort) and heat up the speaker voice coils to the point where they can't move freely inside the gap. You may not notice the distortion, especially if it occurs in a movie during an explosion. Your description fits this scenario.

However, there are other possibilities and they're easy to test:
-- Make sure the speaker wires are "in phase" so the red and black terminals on the receiver are connected to their respective colors on the speakers.
-- Play the radio to make sure you don't have a bad connection on the CD or DVD inputs. If this clears up the problem, change out the RCA cables.
-- Try the receiver with a different pair of speakers.