I think the single best thing you can do to improve the sound you have is to get away from the receiver for all 2-channel listening. I have yet to hear a receiver, even used as just a preamp, that can remotely come close to the sound of dedicated 2-channel preamp/amp setup.
I need to get one of my monoblocks fixed, so I did a little testing with my receiver (an Onkyo 876). The sound was so terrible in every way, I decided I'd rather go without listening while the amps are gone. I'm not exaggerating here. I had forgotten just how great a dedicated 2-channel pre/amp can sound. I continued the experiment by testing with and without my DAC, and using the monoblocks for the power. All of it was still awful. There was no configuration that even came close to what I'm used to hearing.
So my vote would go to getting a nice 2-channel preamp with home theater bypass like the Parasound 2100 (or similar), and a 2-channel amp of your choice. Then use your receiver to power your center channel and surrounds for HT. This is what I do, and it's the best of both worlds.
As far as sources go, vinyl is like cave carvings. It's way too much work! If your listening habits are to play albums all the way through, I suppose it's not so bad. I've been streaming music since Turtle Beach launched the Audiotron (a long time). I can't even spin CD's as it's too much work as well. I love having access to any track in my library in seconds, and nothing beats playlists.
I admit I haven't heard a great vinyl setup, so if and when I do I may eat these words, but streaming digital is the only way to go. I use a Logitech Touch (with some mods), an old PC running as a server in my basement, and a nice DAC. I am VERY happy with the sound I have, and I've heard nothing better. My digital front end was $180 for the Touch, $250 for a DIY linear power supply, $100 for a nice digital interconnect, and about $250 for the old computer with a 2TB harddrive. That's an impressive source for about $800. YMMV. You could still get a great DAC, the preamp with HT-bypass, and a great amp for the $3,500 you have to spend.
For the record, you are only going to take that setup you have so far with the speaker placement you have. But if you want to get the most from it, I'd go for what I stated above without hesitation. You have absolutely fantastic speakers, and they'll step up to anything you feed them on the front end.