I would go with the Lee Jackson preamp, a good cab, a tube power amp preferably and if you need any more gain than the preamp has, then a Lee Jackson gain boost pedal. No active boosts, any active pickups or any pedals.
Because either they'll colour your tone, in a way you can't always know beforehand, or it'll change it drastically.
So the question is - why do you want the GP-1000? For the fact that Alexi has it, or the fact that it sounds good on the albums (wich I guess is where you've heard it)?
Because then I'd go with a good passive, and maybe a Lee Jackson gain boost pedal, or the Musamaailma gain boost if you can get your hands on it (I'd prefer the pedal tho).
I'm not even sure you'll have to shift the pickup, since the HZ with the right amp and settings can get a pretty killer tone.