Adding subs and amp to stock JBL system on 07' Tundra DC

Hi everyone,

I recently got an '07 Limited Tundra Double Cab with the JBL audio system in it. Compared to the stock stereo in my Tacoma, this sounds great. However, I found it was lacking in the bass department and the 55 watt JBL sub was not cutting it for me. I already have 2 10" KICKER CVRs and a ZX1000.1 amp from my install on the Tacoma, and I'm looking to add these to the JBL system.

My plan: I'm building two small boxes that will rest on the floor in the back with the seats flipped up and will be removable if I need to use the seats. The subs are large and will not fit under the rear seats, but I am willing to sacrifice the room. I'm not too worried about routing the power and stuff, but my major concern is the audio signal to my ZX amp. What I'm going to try and do is rob the signal from the existing sub. If I unplug that, I find that there is a mono speaker-level signal for that sub. I'm going to find a way to attach that to some RCA's and set the amp to the high-level input mode, which it has.

My concerns: 1: That the speaker-level signal is processed in some way by the JBL amp and will be poor quality for my amp and subs.
2: Since the speaker-lever signal is a mono signal, I would only be able to hook it up to one of the inputs (R or L) but since this is a mono amp anyways, I'm thinking that this won't matter.

Does anyone know if this will work? Thoughts? Suggestions? I spent a lot on these subs and amp the first time around, and I'm looking for the cheapest way to get them going in this Tundra.

Thanks.

Also, I forgot to mention, the whole reason that I'm doing this is because I would prefer to keep the stock HU. I like the Handsfree features seeing as talking on your cell phone is illegal up here now. The settings in an aftermarket HU for Subwoofer volume not being there is solved by the remote base knob that is supplied with the ZX amp.