+1 for the statement regarding computer-free playing - you need a nice desk :)
would love to do an "a-b listen-comparison test", a without the mixer and b with the mixer in the analog chain :-)
argh, nebula, thats how many busses on that soundcraft? dammn ;-)
by the way, some mixers (like some allen & heaths) have options to reduce the analog audio path from your line signals:
a) preamp bypass (preamps are only necessary for mic-level inputs, not for "strong" synth line inputs)
b) eq section bypass
finally, there often is a "direct out" feature, which allows you to grab your most important channels and capture them unmodified using your audio interface.
regarding old digital mixers - be careful, they may be your weakest element in the processing chain, e.g. if they offer only 44khz/16bit sampling rate per channel.