As I said elsewhere, I don't do much woodworking myself but I recently modded my own console. Mine was sagging from the weight of all the components so I added support; I see you wont' have that problem considering how you put the base together.
However, I'm quite proud of my idea to add drawer slides to one shelf, so I could slide the receiver out. I used those slides
, good for 100 lbs. Since the shelf takes up all the space between the side walls, I simply cut a 2x4 along its length to have 2x2's, and attached the drawer-side slides to those 2x2s. Once in place, I put the shelf on top and screwed it on the 2x2s.
As for the back, I have an idea that I'd like to try one day. The problem with open backs is that you see the wall and all the mess behind (I think I have kilometers of HDMI cables and speaker wires there). If you close the back, you don't see the mess, but you still have to make holes for the wires, which can be hell if you want to know which cable is which. Also, holes are good for ventilation, but not really sufficient either. So I thought of leaving a full 2-3" gap in the back panel for the wires and ventilation; but there could be another offset back panel further to hide the back wall and wires. This would also provide a natural chimney for hot air to escape.
I realize it's hard to picture what I have in mind so here's a quick schematic:
I guess one could also install a fan at the back to force air through the console, and it wouldn't be visible either.