They do two different versions. Mine with the 3 sections, and another with 2 sections. I guess you could get 2 two section versions and butt them up together.
IIRC, the back panel is a single piece. It has holes in the top and the bottom behind the shelf areas to allow cables/plugs through. As I said, I measured the height and width of the amp, deducted about 5/10mm, then cut out the back panel. You can't see the hole unless you pull the amp forward.
There are 4 small holes in each side panel of the centre section. These would have been where the drawer runners would have been fixed. They're only small though and I guess if you objected too much, you could use a small amount of white filler.
Legs that come with it were high enough to mean it would have been almost touching the bottom of my wall mounted TV (TV was already bolted to the wall, centre of screen is at eye level, so didn't want to alter it). I wanted a bit of clear 'air' between the two purely for aesthetic reasons. They do a variety of different legs for kitchen units, they were cheap enough so we bought the shortest they had in a style we liked.
I don't know if you've got an Ikea catalogue handy, or if your near enough to one to pop along and have a butchers at one yourself. I think this would be the best bet.