You will probably get 2 types of general answers on this. Some people will recommend 2 channel only gear to be merged into a multi channel setup like you now have, and others will recommend better multi channel gear. I have no idea why, but the debates usually deteriorate into insults back and forth. I'm hoping that dosen't happen here.
Heres my recommendation. I believe that your biggest improvement in SQ will come in the form of a new preamp. Its an incredibly important piece in an audio system and until you get it right, your system will be lacking. I would take your $5000 budget and buy the best 2 channel preamp you can afford. It should have a HT pass through so you can still use your multi channel system for movies and music whenever you want to. Upgrade your amps at a latter date. Some brands I like are Ayre, BAT, Aesthetix and CJ. There are many other great choices as well. Also, I'm not up on every single feature so just make sure any of these preamps have the HT pass through feature you will need.
Heres my recommendation. I believe that your biggest improvement in SQ will come in the form of a new preamp. Its an incredibly important piece in an audio system and until you get it right, your system will be lacking. I would take your $5000 budget and buy the best 2 channel preamp you can afford. It should have a HT pass through so you can still use your multi channel system for movies and music whenever you want to. Upgrade your amps at a latter date. Some brands I like are Ayre, BAT, Aesthetix and CJ. There are many other great choices as well. Also, I'm not up on every single feature so just make sure any of these preamps have the HT pass through feature you will need.