As others have noted, you have a decent TV and players, so you should be able to get a pretty clean picture.
To figure out where the problem is, I would start by plugging the DVD player directly into the TV set (don't go through the receiver). Pick a DVD of known high-quality and use that for all the tests. We'll also assume your HDMI cable is good; any HDMI cable ever made can send 480p or 1080p video over a distance of one or two meters, so you don't need to worry about spending a lot of money for a fancy cable. But if it's a bad cable, you'll want to replace it.
Hopefully you have a DVD with proper test patterns (even an old disc with THX "Optimizer" or "Optimode" patterns will work). Start by calibrating your set to these patterns. If you can't do this, look for any preset picture settings your set has like "Sports", "Movie" or "Standard". Choose "Movie" or whatever is similar to that. I would also recommend turning off picture enhancement settings like "Noise Reduction (DNR)", "Edge Enhancement", etc.
Your TV has its own "upscaling", so try setting your DVD player to output 480p. Does the picture on your TV look better than what you had been seeing? If not, then try a different HDMI cable (even a super cheap one will work). If that doesn't help, then your TV is having a problem.
If it looks good, then you have a good baseline to work from. Next, try setting your player to 1080p. Now does the picture look better or worse? If it looks better (or the same), then you can hook it up through your receiver. If it then looks worse, then your receiver is doing something to it. If it still looks good, then problem solved.
If you change your DVD player to 1080p and it looks worse than 480p, and this happens with all three DVD players, then there is something wrong with your TV. It should most certainly be able to accept a 1080p input without degrading it.
If this still doesn't make sense to you, or you need more help, then I would recommend getting out a digital camera and taking pictures of what you are doing and uploading them, so we can see the menu screens on your TV and players, as well as the picture quality you are seeing.