Re: active electronics for passive pickups
Bartolini has been building their "actives" like this forever. They bring the passive pickup leads out for you, and you have the option of wiring it passive if you like, or wiring all of your pickups to the switch, and then using the preamp housed inside one of the pickups to make the whole guitar active, with boost options, etc.
The biggest difference I see is that EMG is relying heavily on their preamps for the precise tone of each pickup. So I would assume that if you simply put an active boost on the H4 it won't truly become an 81. I imagine the 81 preamp having specific tone shaping components that wouldn't be present in a straight boost. And their Presence or Expander switches may not get you there either.
IMO if you want to start playing around with pickups that are designed to be active or passive, and you want to play around with resistor/cap networks to fine tune your sound to perfection, it's Bartolini all the way. There's no substitute. Once you truly understand Bartolinis, EMG doesn't even need to be in business anymore. In a perfect world you'd have EMG's marketing team combined with Bartolini products. And you can toss the Livewires in there as well, they seem to approach active pickup design with more of a Bartolini mentality, that the tone is in the pickup, not the preamp.