PFR, what mandoplank did you start with? Mine was an ebay pile of shit, but now I have got the hotrails humbucker doing more or less what I want I'm tempted to start again with a better base instrument, a Mandocaster or Mandobird but I don't know whether to go for an 8 or a 4 ...

Curiously, for me, the best way to sweeten the sound of a spiky mandopiezo (I have a B-band in my Kentucky Flatiron) is with a distortion pedal. It sounds nothing like a mandolin, but it sounds nice to me!

The LR Baggs outboard preamps can be quite effective, but if you get them wrong they make things worse. So long as the piezo is correctly preamped any DI box should have a high enough input impedance and cause no problems.