I belive that the Gain knob you speek of works for both RCA's in and out's. But if they were not getting a strong signal, they need to turn up there gain. If running to DAT you are obviously using the 1/8" line in, in which he/she would HAVE to turn the gain up on the DAT. MD same thing, but I know once you start recording w/ a MD, you cannot change your record levels, this must be done before recording. I could be wrong on this.
I know when I run a 3rd mic into the RCA IN on my UA5, I turn the gain knob up all the way and it gives me between 12 and 6 db's, just with the 3rd mic. Then I plug in the XLR and adjust from there.
Hope this helps, but unless you really care about the "PATCHER'S" levels, I would not worry about it, just my opinion on that one.......
JDS