Help adding amp to 2011 screw

Need some help adding an amp to my current setup. Forgive my ignorance on this. Here's my current setup. Kenwood dnx890hd navi with idatalink maestro interface to factory amp out to kenwood excelon components (front) and kenwood coaxial rears with stock sub.

I want to add a 5 channel amp to drive the kenwoods and replace the sub at the same time. Is there a decent sub that will drop in the stock enclosure?

Big question I have is where exactly do I tie in the amp? The kenwood has a built in DSP so do I just cut out the factory amp completely and take the high level inputs right there and then use factory wiring from the amp to the speakers? Is something like a soundtream Picasso nano 5.640d decent?

I love doing my own work but I'm no audiophile and just looking for some more volume and crisper sound. Current setup is actually not horrible but I know the factory amp is lacking.

you could bypass the oem system all together at this point

So this ended up being pretty easy. I'll post here what I did in case it might help someone in the future.

I picked up an 8" JL microsub after hearing it in a coworkers F150. It's an exact fit under the right rear seat. I mean pretty much the same exact dimensions. You could actually use the factory mounting brackets but I didn't want to screw into it. You're not going to hear it coming 3 blocks away but feed it enough power and the bass is nice and tight. It hits you in the chest and will rattle the hell out of the rear view mirror if you turn it up which the factory sub could not do.

I got a good deal on a soundstream RUB5.800 5-Channel amp which seemed to get a lot of good reviews . After installing it I have to agree. I set the gain with a multimeter using JL's guide and then tweaked it just a bit by ear to my liking. I can safely crank the Kenwood deck to max with no distortion but half way up is all you need. I ended up mounting the amp on a piece of 1/4" plywood covered with some gray MLV that I had and secured to the back wall with some 3" screws into the support that holds the rear glass window regulator. Finished off my sound deadening on the back while why I was at it.

Ran power though the grommet below the ECM on the firewall and then down the passenger side in the wiring channel. 3 sets of RCAs in 3/4" TechFlex (love this stuff vs. standard split wire loom) down the drivers side.

Got lazy on the speaker wiring and ran it from the amp out to where the factory wiring sits in the channel ahead of the doors and spliced there. Didn't want to fool with getting new wiring into the doors. I thought about tapping all the speakers in one location right at the harness after the Sony amp but didn't feel like removing the entire console to get to it. I have the factory wiring schematic here if anyone needs it. Using the color key they're easily found and are all in twisted pairs that make them even easier to ID.

All in all a pretty great upgrade.

dont worry, the oem wire is more than enough. there is no need to run speaker wires through the door.