I am a recent mark iv owner and also new to the board! My iv is later also. I am not dissapointed in the gain. I think the settings are everything in that department. If I were to take a guess I would say you have a tube issue/s. A lot of guys don't like R2 on the mark iv. I am not one of those people though.
For me the tone of R2 is great for what I want out of it. In a way I like the tone (not the gain) about as much as the lead channel. It is just that the lead channel has a lot more gain and compression available. R2 sounds good with a boost pedal kind of scenario also if you want it to have what the lead channel has. For me R2 absolutely needs to have the gain/pull fat pulled and wide open. Also the presence pulled. It still is not metal gain at it's best but it is a good classic rock gain that a boost can push onward. I use it as chicken pickin' slight dirty with guitar volume down/single coils and humbuckers full up should get you cream, free, allman brothers, crunchy rhythm stuff. The lead channel should get you to petrucci gain levels if desired...he used to use those.
I tried the jpc briefly (maybe not enough time spent) but I did not think it had any more gain on tap than the lead channel of the iv. I realize it is more like two lead channels than say the iv with R2 being lesser gain than the lead. This is why I say maybe some tube experimenting.
Just for kicks try this...set lead channel like this--
lead gain 8 and pull fat
treble - 6 or more
bass - zero or up slightly
mid- maybe 4-5, I run mine about 6-7 but I like thick mids the pickups make a big difference here
lead drive 8
I run presence at zero and definitely pulled. you may want to open up the presence more than me because I like it smoother
class a
and mid gain
that should be pretty thick and chewy. petrucci settings are easy to look up, you could try that.
you can mess with the eq a bit but I usually just use it very slightly depending on the room not for the big v mid scoop thing.
Just my 2 cents...maybe that was like $1.50!
