what kind of sound are you looking for? i believe you can get all duncan single coils (actual single coils) with that base as a shop floor custom. there are also some single coil size humbucker options as well
you should be able to order the aps2 with an oval base as a shop floor custom through an authorized seymour duncan dealer
Yeah, the Shop Floor Custom options are your best bet here if the pickup you absolutely love has the bigger baseplate. The APS-2 is a great pickup, but will be quieter than your humbucker if you care about that sort of thing.
Well yes, that will be an issue. I am planning to use my equalizer pedal set to flat with volume at -3dB. When I switch to the HB, I will engage the pedal and it should hopefully match the output level of the HB to APS-2.