I've had my 705 for a couple of weeks or so:
Do not connect the Surround speakers to the Surround Back terminals in a 5.1 set-up. The Surround Backs are for 7.1 rear speakers.
Plug the sub into the Sub Pre-Out, not the multi-channel sub connector.
Adjust the sub level before running Audyssey setup. So play a 5.1 film to get the level correct or whatever.
Run the Audyssey setup routine, it does make a difference. It'll also remove the rear 7.1 speakers for you if they aren't plugged in and remove the listening modes that aren't appropriate for a 5.1 setup, so you don't get sound drop outs when the sound goes to a missing speaker. For example it will remove the Dolby PLIIx mode leaving you PLII instead.
When running the Audyssey setup routine place the microphone facing up, preferably at ear level, preferably not on something soft.
Pre-set the listening modes. For example if you use phono inputs from a Freeview/Sky box, change the Pre-set for the analogue input to Dolby Pro-Logic II (or whatever you prefer). They default to Last Valid if I remember correctly. For example, I have changed them all to what I want, so Dolby TrueHD on the HDMI input is set to Dolby TrueHD rather than Last Valid (you'll see what I mean).
Can't think of anything else for now. Enjoy!