I am also getting a DigiTech RP90 Guitar Multi Effects Pedal today if that can help.
I’ve put some links below to help give you a perspective on what other people say, and the manual if it helps you dial in the tone you’re looking for.
Since I don’t know what type of distortion pedal you have, its kind of hard for me to help you in that regards, but I can at least suggest settings on the amp that might get you as close as possible to a Metallica sound.
The four amp models I would consider would be British 4, Hot Rod 2, Metal 1, or Metal 2. I haven’t used the amp, but that’s where I would start. No effects, Bass 5-6, Mids 3-4, and Treble 6-8. As far as Gain goes, it will depend on your guitar, pickups, etc, but between 6 and 8 will probably be the sweet spot.
Another way for you to approach getting that tone is to use a distortion pedal that cleans up well when you roll the gain down… in other words, if the pedal sounds pretty clean when the gain is at 1-3, then it will work for this… put the gain low (ie 1-3), with the level high (ie 7+). If it has a tone control you’ll probably want it between 4 and 6, if it has a separate Bass/Treble then I would put the bass at 3-5 and the treble at 6-8 (cutting bass before the amp = a cleaner, tighter, more trebly tone, which is exactly what earlier Metallica especially sounds like). This turns the distortion pedal into more of an overdrive/booster pedal.
Keep the amp at the settings listed above, but turn the Gain down or you’ll probably get feedback. A relatively clean "hot" signal pushes an amp (or pedal) further into distortion, so in general it works well with lower-gain amps and amp models. Your mileage may vary (YMMV), but I’m willing to bet that either British 4 or Hot Rod 2 are going to be the closest to the tone you’re looking for. I could be wrong, and it could be one of the Metal channels, but if British 4 = Marshall and Hot Rod 2 = "boutique" (which usually means Bogner or Soldano) then it should be right in the ballpark.
I don’t know much about the Digitech RP series (it’s been a while since I’ve looked them up, and I don’t think I’ve played one more than once), but I’d try googling for the manual if it doesn’t come with the unit already. If you use the pedal for your amp model, turn off the amp model on the amp and turn the amp’s gain way down – you want a very clean sound to make an amp model sound as good as possible. The exception to this *might* be putting the amp model on one of the acoustic settings. Its worth experimenting with, at least. Again, gain at 1.
Hope this has helped!
Saul