Thanks, I'm really liking this rig. The Big J Blades are the Warm & Wooly. If I get the Clean & Clear for my G&L JB, I'll definitely post about them, but the G&L has a slightly darker inherent tone than the Sandberg, because it's alder/rosewood while the Sandberg is paulownia/maple.
I think the Big J Blades do great with the G-K growl. At the show on Saturday, I used the MB-500's boost channel, and got a fantastic thick and slightly growly classic rock type of tone. Two nights later, I played through an older G-K Backline head into an Ampeg cab at a rehearsal studio, and through that rig I didn't like much G-K boost in the signal. I don't know whether that was the head, cab, room, or something else, but it sounded slightly harsh, and dialing down the boost sounded better IMO.
I've tried lots of J-bass pickups over the years, and they've all sounded great through G-K heads and cabs, so I'm not sure how I would rank the Big J Blades in that respect. But I will say they're my favorites out of all the five-string J-bass pickups I've tried, which include stock pickups from Fender, G&L, and Sandberg, and aftermarket pickups from Barden, Aero, and Nordstrand (the Big J Blades and NJ5FS in a Fender). Purely for tone the Aeros are amazing, but the Big J Blades sound fantastic too IMO and they provide vastly superior string-to-string balance.