This is George/Mr Thin
I was looking at your posts last night. It brought back a lot of memories. There is a lot of excellent brain power on this site. I used to go to studios and set everything up when they has tape recorders and record lathes.
I am in the middle of fingering out another website. I am going to post some pictures somewhere so people can see some of my work before I get a new site up.
A few thoughts.
A. Would it make sense to have the summing sends from the channels balanced as to remove the ground reference? I would use a differential receive of two inverting op amps into a differential op amp for balanced receive with summing. The massive ground floor in the console can be a real problem. When I built studios, we did a whole room prep-up before bringing the console in. This included ground planning the floor with copper, a large ground rod etc.. Op-amps are not unreferenced from ground. If you put a signal into the plus or minus input on a differential op-amp, it will have the same output as into both, balanced. In differential mode, you at least can have the CMRR to reject noise and take most of the ground plane effect away.
B. The problem with the bandwidth is that there are noise artifacts, harmonics and instant bursts that last milliseconds that ad harshness to a sound. The reason tube or older transistor or transformers add sweetness is because they will not pass very quick bursts of transit energies that harshly color the sound. So more bandwidth for this is not maybe a good thang...
C. I have a friend that got into Pro-Tools. There have been a few discussions about throwing out the mixer and just having a line summing system. I designed out a tube summer that would have +-6DB level adjust and pan per channel, pin 2+ drive I was going to make the pots with two photo-cells driven by lights so the whole thing could be automated.
D. The final thought and what I have been pestered about is doing it with 12au7's or 6sn7s at least in the output buffer so the line summer would be a Phat making deal. Using tubes to Phatting up the sound.
For some 5 or 7 band EQ's I built using 6DB/octave filters, I designed a 12au7 summer to add the bands together that works fine, no effect from other inputs.
Then a low impedance summer so the sum resistors are not to big and normal isolation between inputs.
I recently have been making mike preamps with these Yamaha PM-1000 boards that have two Phat little four-trans amp that are the building block amps for the whole console. They are single rail, output cap amps, but real phat with an open sound. They will drive down to 30 ohms at high level. A older style trans circuit could be used to get low impedance out without the trans.
E. You might consider having a Phat circuit before output that could be faded in/out for tone color. Even fading paper-oil or poly caps in/out for tone color.
I also have used simple resistor/capacitor networks that are slow, between stages to add color.
F. The last is the balanced out. When I worked at Sound Image, being the repair tech, I found there is no way to know for sure what any piece of gear is doing for the in/out wiring. Some gear is only pin2+ and 3 just hangs. Some have differential inputs and transistor buffers to pin2+ out. There was even some gear that was pin3+ and the cables to this gear had to be reversed..So the balanced out scheme is cool, but what is the input structure of the driven device?
I was building acoustic Git preamps for a frind for research and we found the input on his Sony mixer/HD recorder thang has a 1.5K resistor between 2-3 into a diff op-amp. We found a lot of the cheaper mixers do this. We were getting weird responses because of the load to the final tube stage in the preamp. Some mixer companies just assume a 1.5K load for line and mike.
Again though, what is the purpose of the mixer if not to add color???