Then any delay (for example) is then going to get distorted by the preamp in your amp. Maybe that's the sound you want, but generally you want delayed distortion, not distorted delay.
Guitar > GT-10 or X3-Live > amp preamp > GT-10 or X3-Live > amp power amp
The send receive in your GT-10 effects path should be AFTER your comp/distortion/amp sims and BEFORE your chorous/delay/reverb effects.
Note that you can still put external pedals in the right places - i.e. real wah pedal before the GT-10 input or a distortion stomp box before the amp input and other effects processors in the loop before/after the GT-10.
Here is a link for further description: http://www.benvesco.com/blog/pod-x3-live-tips-and-tricks/2008/pod-xt-x3-live-tips-and-tricks-four-cable-method/
