Wow... thanks for your help, Ian. I see how building with your TLA's could simplify this project a lot. I hope you sell PCBs?
Now, the switches on the auxes will obviously reduce cost, but is that the only reason for your suggestion? If so, I would prefer to put a pot for each aux on every channel.
Perhaps I should explain my thoughts around this project...
I produce electronic music and will be using the mixer in the composition/sound design process. I rarely mix more than 4-6 synths at a time, but I like to use full channels for FX returns and use the auxes to "chain" effects. For example, I might send a synth to a flanger on send 1 and to a delay on send 2, and then decide to send the delay return channel to the flanger as well, so the repeats will go through the flanger twice. It's a creative thing and it's important for me to be able to experiment without having to rearrange the FX sends.
It's also worth noting that many of the effects I use are actually guitar pedals, and most of my synths are somewhat noisy, so for the above scenario I'm not overly concerned with noise. I also plan to use the mixer for summing during mix down, and here I obviously do care about noise.
The idea is to add "summing" switches to the channels that bypass faders and pan pots, and sends the signal directly to the stereo buss at unity gain (left buss for odd channels, right buss for even channels).
I would use decent (but not super high end) faders and pots, decent output transformers for direct and aux outputs, and then invest in some REALLY nice transformers for channel inputs and stereo out.
This way, I can have a ridiculous number of aux sends AND high end summing in a single mixer without breaking the bank
(Well, I might break the bank a little, because those hight end transformers are EXPENSIVE).