The channel inserts are designed to use a TRS connector. (3 conductor 1/4" connector) The send of the channel is usually on the tip and the return is usually on the ring. When nothing is plugged in, the tip and ring terminals are connected and the channel behaves normally. By interrupting this connection, you are able to bring the signal out, route it through outboard equipment and bring it back in using the same connector.
You could connect your preamp to this jack but you'd have to either use a specially made cable that had used the ring terminal of a TRS connector for signal, or you 'might' get it to work by plugging in a 1/4 cable half-way (when you feel the first click) In either case, it will be an unbalanced connection and there is a higher possibility of added noise or interference.
Another thing you can try is going into the AUX return inputs. These are really just additional channels with no trim or EQ.
If it was me, I'd just use a regular channel and make sure my EQ was set flat. Oh, and you should be using the balanced line input. You don't want any additional coloration from the mic preamp circuitry.
post edited by Rick O Shay - November 02, 11 9:14 PM