Wei Val... it's not the length that matters, but the technique FTW!
EQ... go through the FX loop and bypass the pre-amp!
The pre-amp, power amp and cabinet will all colour your sound. So when you run your GT through the AVT pre-amp, you'll be dialing your tone to take into account those 3 things, i.e. you'll be compensating for the AVT's pre-amp, power amp and cab.
When you swap amps, you'll have a bloody headache trying to figure out which parts of your tone came from the GT and which came from the AVT... e.g., you've got a kick@ss boomy sound. Is it from your GT settings or the AVT?
My caplang practice amp has no friggin FX loop so everytime I set my tone at home, I'm always going through the pre-amp. (And for a while I was using the dual pre-amp thing on the GT... so 2 GT pre-amps through amplifier pre-amp... = just shoot me.

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The trix for me is to always have the output at Line/Phones so I've got access to the cab and mic sims. Then I balance between the GT pre-amp's direct level (i.e. no cab/mic sim) and mic level (i.e. with cab & mic sim sound). The end goal is for me to be able to get the same tone consistently... whether it's good tone or not is a different story lah! (consistently crap tone mou...

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Anyway, I find that through "coloured" amps (especially Marshalls!), it's better to use more direct level whereas through amps like Hardcore Max half/full stacks, more mic level sounds better. YMMV.
Generally, the direct level will be very very sharp and fizzy (since it expects you to have a physical cabinet to soften up the high end) whereas the mic level will sound rich but a bit boxy, coz it's like having a (modelled) cabinet within a (physical) cabinet.
So when I swap amps, I can pretty much tweak the direct level/mic level balance to get back (most) of my home tone. And then tweak the global EQ a bit to compensate for volume.
Bottom line for me is having an easily tweakable tone... a portable tone lah. Essentially one that is not so ub3r-EQed (no pun intended!) or FXed so that everytime I plug into a different amp, I don't have to rebuild the tone from scratch...
Not sure about the volume being lower without the GT pre-amps on though. But it kinda makes sense because when you've got the pre-amp on, you are amplifying the signal...
...sorry for the short post.
This post has been edited by RustReaver4D1: Nov 16 2007, 04:10 PM