Not sure how you calibrate or what white noise is, I am getting a svu-2 so maybe I will understand more when I get it. I think you are be right about running out of the jb3 though.
ok what you do is make a mp3 of "white noise" and set it to be 0db
you can do this in programs like cool edit pro and such
then you play the white noise and "record it"
while recording it you can use a small screwdriver to calibrate it (there are little screws on the back you turn)
and you turn them untill the levels are at 0db
you now know that the svu is calibrated to be exactly 0db when it lights up to 0db
otherwise if the lights goto 0db it could really be recording at +3 thus clipping or on the flip side -3
if you are lucky its right no like mine was but others are not so its good to test it out
there is a lot more on this in the team svu thread - which while i was typing this got linked above