So what kind of sub are you running because I’m trying to do the same as you did so I don’t have to replace amp and wiring. I have been looking but a lot of subs require about 150 rms minimumI went to my local dealer, and they provided me a breakdown of how to do it.
I am NOT going to to replace the stock 8" DVC. I installed a 10" DVC in a ported box, using the factory power, WAY more bass. I highly suggest to you to look into it. I am going to simply remove the sub so the sound can pass thru the read parcel deck. I was looking to install my Dual 10" MTX Box with my Boston Acoustics GT-22 AMP, but once I added the sub box, I found that (for now) it is ample for my veracious appetite for bass. Don't get me wrong; I WANT more, but surprisingly, the stock power is enough, if you are willing to add a sub box.
PLEASE NOTE: I had initially added my 2 MTX 10's (sealed) to the factory system, I simply connected the factory sub power to the box. It was BETTER than the factory 8" by far, but wasn't enough. I had another single 10" DVC sub, in (this is important) a PORTED and tuned box, and THAT made a HUGE difference. Running all the stock power (two lines) into the 10" DVC and THEN into the ported box gave me adequate bass.