You can run some subs off of it. If they still build Rockfords like they used to, it will have plenty of balls. One of the more impressive systems I heard back in the day used a RF Punch 150 to run the dogshit out of a pair of 15's. That said, do not hook up a 2 ohm load to each bridged side of the amp unless you are absolutely certain the amp is still rated for 2 ohms when bridged down from 4 to 2 channels. Not many are unless they are also rated for 1 ohm with unbridged channels. When two channels are bridged into one, that new single channel effectively "sees" half the impedence you are presenting it. If you put a 2 ohm load on a bridged channel, that channel will see a 1 ohm load. It might work if you were able to keep it cool enough, but I wouldn't recommend it. Some amps are built specifically to handle 1 ohm (or lower) loads, but I'm not sure if that amp was built for it. Not to mention your damping factor usually drops like a rock with the really low impedence loads. You get more power but less control over the sub. I cheat and run a single JL 12W3D6 with both voice coils paralleled for a 3 ohm load. The factory Kenwood MSP amp is bridged (and therefore seeing 1.5 ohms, just below what it's rated for) and provides a shocking amount of output...it sounds awesome...but it runs hot as hell and it's a miracle I haven't killed it yet.
A 4 ohm 10" sub on each bridged channel would give you a nice, safe setup, and I bet it would hit pretty hard if you choose wisely on your subs and get the enclosure right. If you're not wanting monster "wake the neighbors" bass, I'd go with a good single 12" DVC sub (hook one voice coil per bridged channel) and keep some trunk space. My recommendation would be JL Audio subs in a sealed enclosure unless you have room to go with a ported box and tune it right. l still haven't heard anything that impresses me more than a JL, and that's why I've always come back to them for over 10 years now. Good luck with it.
*edit* I checked Rockford's website for the power specs. It is not rated for 1 ohm operation. 4 ohms is as low as is recommended when it's bridged.
<TABLE cellSpacing=0 cellPadding=0 width="100%" border=0><TBODY><TR class=highlightOn vAlign=top><TD style="PADDING-RIGHT: 6px; PADDING-LEFT: 15px; PADDING-BOTTOM: 7px; VERTICAL-ALIGN: middle; PADDING-TOP: 7px" noWrap width=235>Rated Power</TD><TD style="PADDING-BOTTOM: 7px; VERTICAL-ALIGN: middle; PADDING-TOP: 7px"><!--ProductFeatureValue-ID:1241-->100 W x 4 @ 4 Ohms RMS
200 W x 4 @ 2 Ohms RMS
400 W x 2 @ 4 Ohms bridged RMS
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Dude, if this thing will put out a true 400 W RMS x 2 when bridged, I'd sure as hell use it for subs! Another option would be to run two channels in stereo to your front speakers and then bridge two for a sub. You can use it on your smaller speakers just fine, but 100 true watts (or more if it's underrated) is a lot to be using for your full range speakers, and you better have them crossed over to keep the sub bass out. It would almost seem a waste to use an 800 watt amp only for your full range speakers. Just my .02