I don't have any tone loss, the signal is just a little low compared to plugging straight into the amp...I keep the volume on the RMP all the way up, all the time...You might try to flip the ground switch on the RMP to see if any noise/hum goes away....
I keep my track fader the di is on at "0", & the pan centered...If you lower the fader, your signal will lower also...Same thing with the pan, instead of the signal hitting "up the middle", it'll be lop-sided, & the result will be a lower signal going into the RMP....
I don't know anything about Sonar, I use Reaper, but there should be a way to get your signal level to the same as if you were plugging straight into the amp...I've done a lot of test/trial runs, & with my setup, I have to bump the hardware output (routing in Reaper, which sends the di signal to the RMP) about +3 db...YMMV, as another member here using the same daw, re-amp box, but different interface (I think), has to lower his signal a bit...
Not sure, but you may need to get another interface that has more outputs than the 2 mains, my interface has 1/2 or 3/4, I use 1/2 for my monitors, & use 3/4 (actually just 4, using a TRS to XLR cable) with mine...The reason I mention the extra outputs, on my interface, the main volume knob controls the output to 1/2, if I had to use these, I'd have to find "the spot" on the dial to match what the interface is putting out, to what a guitar would....
Here's a pic of the back of my interface (which at the time I bought, I didn't even use real amps, & had never thought about re-amping...I lucked out on this...):
I've also read that a shorter cable going from the RMP to the amp helps, but I've never had any problems using a 10' cable myself....
Hope this helps...