That sounds like the jb3 to me. You say you normalised the recording... how loud is it actually? From my jb3 I get a constant low level noise, plus a 'tick' every second or so, which I assume is data being written to the hard disc. In my case it is VERY quiet, you need to apply a LOT of gain when mastering to hear this.
I guess, since you are using a preamp that you don't need to apply much gain when mastering? You could try this as an experiment... instead of using the mv50 to give all your gain, use the jb3 to give some of the gain... so... for example, set the jb3 to +6dB gain, and run the mv50 6dB less. Should give the same overall gain, and will now exercise a different portion of the jb3 analogue input. That would be interesting to hear how that compares.
Another option of course would be to run an external a/d converter...
best regards,
stephen