Another thing you may want to consider is using linear pots on each channel to create a left and right out - a pan function. I'd suggest using dual or quad opamps in DIP packages. Check audio projects on the net for recommendations. Analog devices makes some good stuff, as well as many other suppliers. Suggest using bipolar transistor opamps: they are typically, not always, quieter. Easier if you create a bipolar supply (+/- 15V): that way you don't have to DC offset your opamp inputs and capacitor couple inputs.
Suggest buffering each input and feeding low impedance opamp output to a pot and/or pan pot and then summing using another opamp. You could gain up each buffered input channel to something resonable like 10 and use pot to attenuate to desired level. As long as you're building it, why not have multiple independently buffered outputs for use on multiple amps, unbalanced out, etc. You could add a high cut cut if you want.
See if you can find schems for simple 4 or 8 channel mixers to see how they implement circuit functions.
I don't really need a balance or pan pot, just because of the nature of how I'm using it, but that is otherwise pretty much what I'm doing. ±15V PS, buffered inputs with the level control after the buffer, and a simple summing amp. If I was going to do any kind of flitering it would very likely be a high pass filter, as I tend to like quite bright delays and such (as a recording engineer, I can get a lot of nice brightness without making things sound shrill by putting a low pass filter on the feedback. One of my favorite tricks.) But for this, that probably won't be happening.