Well - there's some truth to what the guy at GC told you. Preamps like the GT-8, Line 6 POD and other "Direct" boxes already attempt to emulate the compression and EQ characteristics of the amps & speakers they're modeling. As you may or may not be aware, guitar-speakers are actually *designed* to roll off much of the HF and provide a strong boost in the mids/high-mids.
For example, the Celestion "Blue" speakers found in an AC-30:
Amp modelers try to simulate this effect via corrective EQing (among other things) and running this signal into a *real* amp can cause some muddyness/uglyness that isn't really useful for anything other than "effect" - esp. if you're planning on running it into an amp with a *very* distinct character like the AC30.
Now - what your GC salesman DIDN'T tell you is that units like the Boss GT-series stomps and various other boxes like it *CAN* have different sections of thier programs bypassed. For example, you can use the 'stomp-box' modeling and the effects, yet bypass the speaker emulation if you choose to run this as a front-end to a real amp. I strongly suggest checking out the manual for the GT-8 to see how flexable the unit is in this capacity.
As a final point to note: While the AC-30 is an incredible amp and a true studio classic, it's *not* a main-stay of modern-rock distorted sounds. For a comparable price-tag, you should check out the Mesa/Boogie Rectifier series or the Marshall JCM2000 DSL line. Both are the current standards in high-gain rock tones.