Summing mixer & monitor controller

I've been thinking about the possibility of making a summer mixing/monitor controller; basically the master section of a mixer.
I don't really want to get into designing stuff, so I've been trying to put this together with readily available parts and keeping it as simple as possible.
Here is the basic signal flow:



I plan on using Classic API's 2-aca-bo for the summing section, with an insert point and a stereo fader between the 2 halves...
All the switches would be goldpoint, with the Mode select switch following this:
http://www.goldpt.com/ffms.html

JLM's kits for the VU buffer and headphone amp (x2).

I thought it would be cool to have it in a desktop console-type box, so have been looking at something like this:
http://uk.farnell.com/jsp/search/productdetail.jsp?sku=3537020


The PCB's should fit comfortably in the case, although there doesn't seem to be much room for a power supply (perhaps this should be external though?)



I knocked up a quick front panel layout...looks a bit sparse to me..



Anyway,
Can you see any potential problems here?
Should I be buffering/transformer balancing the outputs - would I gain an advantage?
Is there anything I should be including thats not there?

I've not paid any attention to the impedances of the various sections, but I don't see how it would be a problem...