acidrock:
If you're referring to the input on the front panel of the guitar amp, I would suggest that you are not going to get consistent sound and that you are not going to get the same sound as you do from the headphone outputs on the pedal. It's not that it's not workable---it's just more work than you need to do. It also limits the portability of your sound, because now the front panel of your amp becomes necessary to reproduce your sound (i.e., the sound becomes a combination of your guitar preamp+ pedal). The reason is simple. Modeling pedals are preamps. Even the "DIRECT" setting in a Digitech pedal is a model of a staight wire, not an actual hardwrre bypass. Running a preamp into a preamp, well---imagine getting a Marshall sound by running your Marshall line out into your Fender guitar input. It's going to be more complicated that way than bypassing the Fender preamp and going directly into a power amp. If you want easily adjustable consistency between your live sound and your pedal's sound, I'd strongly suggest an insert/aux/line input rather than the guitar input on the front panel. Mixing boards and keyboard amps, generally, will not have this problem---their inputs match up better with the outputs of modeling pedals than do the instrument inputs of guitar amps.
There's no "one way to Rome", that's why good modeling pedals have a "What are you plugging into / Target System" feature (I don't know if the RP300 has this feature). Even when you select something like "guitar amp input", you still have to deal with the front panel EQ and interaction of channel volume and master volume to set your sound. This added work is cherished by some ("I can ttweak the sound") and disliked by others ("Too touchy---doesn't sound like the headphones/recording). If you log onto DigiTech GNX, Roland GT, Line6 POD, or Behringer V-Amp websites and register with their users' groups, you'll find that a common questions id "How do I set the XXX to get a good live sound?" If you search these forums with keywords like "live", "setup", "stage", "effects loop", "inputs", you may see how ubiquitous the problem is.
If you want to match your live sound to your recording/headphone sound, reduce noise, simplify EQing, and allow for painless volume leveling, I would humbly suggest that you bypass the "guitar" input on the front (or top) of your amp. I own/owned a POD, V-AMP, GT-6, RP2000, and GNX2 and have been using modeling pedals live for over two years now, with Peavey, Marshall and Behringer guitar amps (and Mackie, Yamaha, Soundcraft, Ashley and Behringer PA boards). The GNX2 is what I use most often on stage---primarily because of it's built in floor switches and expression pedal, and because of my long-standing familiarity with it. Every one of these pedals has something good to offer. Not everyone of them is best used as an "effects only" device. Complexity was the trade-off when modeling was added to multi-effects pedals.
Still, what a great time to be alive and playing.
Good luck,
Paj
8^)