Looks like the Gustard has two sets of line-outs. If you want to keep it simple, just connect the subs directly to the extra line-outs. If I were you, I would be looking at a couple SVS SB16-Ultras or Rythmik F25s. If you can hike the budget a bit more, maybe a couple of
Funk Audio 18.0s.
As for timing, you will need something to measure the response, so you can set the phase to get the subs time-aligned with the speakers. But if I were you, I would set aside preconceived notions of a simple electrical signal path and arrange a way involving bass management. The problem is, your maggies do strain at bass frequencies when high dynamics are needed, so they generate distortion, and this distortion creeps into frequencies well above that bass range. By using bass management, you alleviate the Maggies of having to deal with the high excursions of bass frequencies, and it does everything else cleaner.
If you want something simple, get one of
these or one of
these. The miniDSP would be great s well, especially when used with a UMIK mic to gauge the response.
