Consider the following:
Boss GT multieffects processors -- inexpensive and easily programmable, and frequently used with outboard distortion/gain units like you have. Very easy to program, with a multitude of effets. In my opinion the best value. The GT-6 has MIDI capability, but I have no experience with it, so don't know how easy it is to program, nor how well it works. Just as an aside, I think my GT-3 is the best multieffects unit I've purchased. It is very easy to control live, but since it sounds like you'll be using MIDI to control it, that may not make a difference.
Line 6 POD -- Effects, but more of a modeler by design. It is also MIDI capable. The POD Pro (rack mount unit) has balanced XLR in/out's. Not easy to control live, unless you buy the extra foot controller, which is quite nice, but you'll be using MIDI anyway. May not be easy to set up with outboard distortion/gain units
Digitach Artist -- no experience with this rack mount unit, but is MIDI controllable, and designed to be used that way. May not be easy to set up with outboard distortion/gain units. Now we're getting up there in price.
Lexicon units -- you said you didn't want those.
t.c. electronic -- very nice units, sort of the "cadillac's" of guitar effects processors. There are two units, the G-major and the G-force. Designed to be controlled via MIDI. Again, I'm not sure if they are easy to set up with outboard effects. Also at the top end of the price scale.
There are some other units out there. I suggest you download the manuals and check out the MIDI functions since that seems to be important to you.
As for the balanced signals, in order to take advantage of it, you'll either need to have effects units that process a balanced signal, or have transformers before and after those that can't, and I'm not sure how well those work. Keep you unbalanced lines as short as possible, and maybe consider getting a backup wireless unit with a different frequency to use in case of interference.
Too Laid Back -- Vocals, Guitar, Synths
Shakie Ground -- Bass, Backing Vocals