2011 accord ex-l
 

what size is the stock subwoofer?

8".... and somewhat pointless ;)

If you want some output, an Image Dynamics IDQ 8" with a modest amplifier (300W is enough) would give you plenty more "ooomph". The trick would be sealing, stiffening, and deadening the rear deck so it didn't rattle like hell.

ID subs are built for free-air (infinite baffle) installations. You get some cancellation from the rear deck speakers sharing the same airspace. If one moves more than the other (sub vs speaker) then the weaker driver suffers.

thanx

Additional Sub
 

I want to add a box in the trunk to compliment the factory system with a bit more low end. Where is the amp located for the factory sub so that i can tap into the preamp signal? I have a 2011 Accord Sedan EX-L V6. Any suggestions?

^^ me too i was going to replace the stock one with a bigger one.

if you add a sub, remove the stock sub. otherwise the bass pressure in the trunk will distort the factory sub anyway. you need a pathway between the trunk and cabin for bass. a box in the trunk will perform based on it's location and orientation.

not sure on the factory amp location. typical locations are behind the glove box or in the trunk...

your amp is behind the glove box.
how do you intend to provide the amp with the sub signal? note there are RCA level signals feeding the factory amp. just not in a nice RCA format. keep in mind the lower level bass rolloff as well the factory unit is known for.

Typical place to get a signal is a LOC with auto turn-on fed from stock sub wiring. Leaving the stock sub disconnected will just hinder bass response.

I believe this is where you can get in trouble with this vehicle and the premium HU. Without disconnecting either the internal mics used for the ANC or cutting the leads to the internal amp marked ANC, he may get a annoying hum from the added sub. Since this HU is actually a Pioneer product with balanced low level signals feeding the factory amp, the best way to do this is to grap that signal, solder an RCA connector on it and run it to the amp. Clean and without the added ANC. Anything after the factory amp is going to have the ANC signal added to it. It has also been suggested that adding a MATRIX or like line driver to boost the low level in an attempt to counter the bass roll off at low volume levels.