I am considering trying a ported enclosure for my two IDQ12s at some point
in the future. However I was wondering if there was any way for me to
hook up a sub sonic filter that uses RCA connections in my situation.

I currently have an MTX404 getting its rear (sub) signal from speaker-level
inputs which is powering my first sub. My second sub is powered by an
MTX251d which gets its signal from the sum-out RCAs of my MTX404.

The only inexpensive variable ssf I've been able to find is he
http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WD1V

....which seems to use RCA connections. Is there any way for me to use
this considering my MTX404 is getting its sub signal from speaker-level
inputs?

Thanks in advance,
Don