TRS = Tip, Ring, Sleeve and is a three conductor cable connector. Pushing one of these into a console's insert jack as described above is an old trick. Since the insert jacks are typically unbalanced, it should be OK. The advantage is to bypass the internal mix preamps.
And that is the issue. Preamp to preamp is generally not a good idea and with the possibility of 48v phantom power going from the console mic input to the output of the external preamp.... well, it doesn't make sense to use the XLR input unless one would have a very good reason to do so. A for -10/+4 that depends upon whether your equipment is professional or consumer level. In general, it's a good practice to start on the lowest setting (of whatever) and bring things up until the desired level is achieved.
