So is putting the stomp boxes in "LINE" switch, will that basically make them like a rack effect? To clarify, I have all 3 of my stomp boxes set to In LVL to "LINE" and the Out LVL to "LINE", and i have the Marshall parallel mix button set to -10db….because if using the stomp boxes in "LINE" mode makes them behave like a rack effect, I'm assuming i would need to set the Marshall parallel mix button to +4db ? 

Yes. You should try that.