@Bobthedog
Effectively you're not gaining anything from going via a preamp and in over mic level vs utilising your Scarlett's DI (instrument level)....other than the tonal differences/abilities of said preamp of course.
I would argue that recording a passive bass in via DI to the interface, which might result in a lower volume level (not being boosted by a preamp), is easily fixed level wise in the mix, and so it shouldn't really be necessary to use a preamp purely for the reason of boosting volume (unless it's REALLY quiet, and that could be a fault?!). That said, if you specifically want the tonal shaping abilities or characterises of that particular preamp, then go for it!
As has been said before, if you're going in via 1/4" jack from the MXR you should be set to Instrument level (or 'Inst'), if you're going in via XLR, it doesn't matter as the interface is hardwired to mic level when using XLR.
Anything else, let me know.
Cheers
Si