hi jman,
i wouldn't bother w/a plate amp; crappy electronics will preclude wanting to cross the monitors over; one of the main benefits of subwoofin', imo... inexpensive pro-audio x-overs from dbx, behringer, art, etc, are awailable from places like fullcompass.com, which are more than transparent enuff to make them worthwhile to use, 'til ya can save for a marchand/ db systems/bryston/etc... and, if ya don't wanna buy a used amp, pick up something like the art sla for <$200...
the above, while a li'l more $$$ than a plate amp, is a *lot* more quality, imo...
doug s.