How efficient are the enclosures?
If they're 8ohm you end up with 8ohm again (series-parallel), but you're not gaining anything except cone area, compared to using one driver on its own. If you have a horn-loaded enclosures (bass bin), they can be very efficient and you should get quite a lot of welly from even just one speaker, unless the room is huge.
Personally, I'd keep two drive units, put them in proper enclosures and use a two channel amp to drive them (parallel-up the inputs to a stereo amp, probably). Phase them carefully and put them facing each other across the room. You could probably fund the amp from selling the other two drive units (or keep them as spares)! You can disguise folded horns as cupboards, coffee tables or even bench seating - an opportunity to be creative!
I assume 'plate amplifier' is Hi-FI-speak for an amp in the back of the speaker? If so it's fine as an idea, especially as it keeps the low-Z speaker cables short, but they shouldn't need to be terribly massive, as long as they have good enough PSUs to deliver the necessary peak power. 500W (RMS/8ohm) would be huge overkill.
Beware the 'power creep' of the spec-writers: ONLY RMS power into a specified impedance (in the specified frequency band) means anything for comparison purposes. "Peak power" is both deliberately misleading and practically impossible to define! I've done outdoor band PA with 2x100W RMS MOSFET amps, because they had well-made conventional power supplies. You really shouldn't need to pay for more power than that for this application.
The only issue I can imagine with active sub-woofers is the danger of ground loops, etc. The only active monitors I've ever used have balanced inputs, so the problem doesn't really arise, but with subs (which, you'd hope, would go down to around 28Hz-ish), you need to be vigilant, and a long cable run at -10dBV (domestic 'line' signal level) might be vulnerable, especially if it runs parallel to a mains run. Watch the earthing and the cable screening (not the silly-price 'interconnects', but anything with a decent braid will do at subwoofer frequencies). You might also put a low-pass filter on the amp input, say -3dB at 250Hz, just to kill anything picked up in the cable - the surround processor won't be sending anything above that.
HTH,
E.
