I have a 2000 Seville SLS, with Bose. Still has three Bose dash speakers, four Bose door speakers, a Bose HU and Bose amplifier (I know this for a fact), yet, the previous owner swapped out the Bose sub for a "Pyramid Royal Blue"
sub. It is a 4ohm sub, and does come from the Bose amp. How can this be? Wouldn't I be able to add say, two 12" subs in the trunk? I suppose not, since their would not be enough power coming from the stock Bose amp.

My question then is, how could I keep my 7 current Bose speakers (I like them) but still add subs one - two 12" subs to the system?

Alright, I thought it over and will just ask - is there a way to split the signal coming from the head unit, and run two different amps at the same time? (Ie, one wire going to the stock Bose amp running all the speakers and deck mounted "Royal Blue" sub, and one wire going to an aftermarket (say.. MTX) amp, and powering either 1 - 15" or two 12" aftermarket subs) I like the Bose system, I just want more bass, and I don't want an entire new system

yes see a car audio shop that can be done no problem