Mid sweep is basically like the selectable mid frequency on the Bartolini preamps, except that instead of having 1 or 2 options to choose from a mid sweep control has a pot that 'sweeps' the entire frequency range so you can chose ANY frequency that's in between the range of the control.
For instance, a switchable mid frequency preamp might let you choose either 300hz or 800hz for the mid control. A sweepable control will let you select anything in between 300hz and 2000hz with just a turn of the knob. That's just an example.
This is the preamp I used:
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This looks to be the MM pickup for the 5 string:
http://www.bestbassgear.com/bartolini-dl5cbc-pickups.htm
The 59CBJS (neck) is the neck pickup I used:
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