As Best Buy/Magnolia was recently blowing new VSX 53's out at $499.98, $600 is way too high for an open box. I agree, $400 would be a good price and no more.
Go back to the Best Buy that has the open box unit and tell the salesperson the above. Have that person verify the selling price of the new units back late Feb/early March through their computer. They will be able to do this. Then make your $400 offer. Also make sure the remote and MCACC mic are with the unit. If not, offer $320 as these items are about $40 apiece through Pioneer. Follow the above advice and you should be able to get an $1100 AVR for $320-$400 with a full warranty! I have made absolutely ridiculous offers on gear at Magnolia and had them accept. Good luck.
Parasound 5125, Parasound 2100, Pro-ject RM 5.1SE, Dynavector 10x5, Pro-ject Phono Box S, Pro-ject Speed Box II, Arcam FMJ CD17, Pioneer Elite VSX-52, Pioneer Elite BDP-52FD, Vienna Acoustics Bach Grands, Vienna Acoustics Theatro Grand, Wharfedale Diamond 9.2s, REL T-1 & Panasonic TC-P54S1