Plugs and Jacks
Just to hit this again:
Generally (and oversimplifying a bit), when you take a balanced output on a TRS jack, and plug a TS plug into it, you'll ground one leg of the balanced output (the R). So, instead of having:
(i) a signal voltage that varies between 0 and 2x (where x is the maximum voltage on a particular leg) -- it's 2x, because when one leg is at +x, the other is at -x;
you will instead have:
(ii) a signal voltage that varies between 0 and x -- it's just x, because all you have is one leg that goes to x, and ground.
The bottom line is that you've cut the signal by about 6 dB.
This isn't the same thing as changing from +4 to -10 nominal level. The difference between +4 and -10 is 11 dB and change (it's not 14, because +4 and and -10 are referenced to different levels: .775 and 1; you know, the dBu vs. dbV thing).
But -- as is the case in some, but not all, situations -- you can work around the whole nominal level issue by adjusting the gain in any event.