where to start....
rockford USED to be nice stuff. they're current stuff is not made very well and is actually pretty ugly if you want to go that far

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with that said, i would do one of two things: 1 sell the rockford amp to a friend. 2 keep it and run one 10" sub off of it. you don't really want to run only one set of speakers off an amp and the other off the headunit. the unamped speakers will clip way before the amped speakers will. you probably won't notice it unitl it's too late and you've blown your speakers.....something to keep in mind.
you sound like you're trying to do a budget system, which is totally cool. everyone has a different budget. budget doesn't have to mean bad though. your rockford amp is decades better than the Boss you are talking about. you would be better off running one nice, properly matched sub off your amp than trying to power two crap subs off that amp. additionally, you would be better running one good sub off your amp than buying a crap amp and crap subs. people think they need two subs for good sound....not sure where that comes from.
one of these Alpine 10" SWE-1042 subs would be better than two of the sony subs you're talking about using. there is much better on the market, but one of these alpine's would be a perfect match for your rockford amp.

the boss amp is cheap and is not going to put out the kind of power it claims it will. it will also starve those sony subs (which don't sound good in the first place) of power if you try to run one off each channel. don't underpower a speaker just so you can have two. you can damage a speaker by underpowering it as easily as you can damage it by overpowering it. you could bridge the boss amp to get 600w rms @4ohms, but the subs would have to be dual voice coil 4ohm speakers to run both of them @ 4 ohms to the amp.
if they aren't dual voice coil 4 ohm, these Kicker KKR 05CVR104 10" would be a good choice. they will pound the sony's into the ground.
bottom line, keep the rockford amp. bridge it and run one 200watt rms sub off it and call it a day. or, run the boss amp with two of the Kicker subs and ditch the sony subs.