The JB3 hard drive noise is there for mic-in or line-in - it's just that mic-in has such severe high frequency rolloff that it's unacceptable for music (but ok for voice).
The hard drive noise peaks at -73 dB when using line-in, gain 0 dB. It peaks at about -62 dB when using line-in, gain +12 dB, so you can't use the analog gain in the JB3 to 'boost around it'. BUT, -73dB is really far back there. I looked at a couple of audience recordings I did and even the quietest moments between songs have average levels of -50dB and peaks of -30, so this is well below the noise floor. Bottom line - if you are getting adequate levels on the JB3 meters, you will never hear the hard drive noise. The reason lots of people are hearing it is because the average microphones can't properly drive a line level input and they end up using tons of amplitude boost in software to bring the levels up (along with the hard drive noise). I have a show where the noise is bothersome during quiet passages, but I had to use +12 dB while recording and +23dB in post...that's a lot of gain!
So if you're getting a feed from a soundboard, there won't be any issue getting adequate levels - I'd use a direct connection to the JB3 rather than the combo of boost-cut-boost (board>attenuators>AD20>JB3). And the JB3 gives you a 24dB analog window on the line-in.