How to improve my system?

Car audio rule number .

Your front speakers make 80% of the soudn you hear, so get the best front speakers, and the best quality install form the before you go looking for anything else. So go out there and start listen to as many speakers as you can and learn about how to install them properly (placement, sound deadening, MDF baffels etc etc).

Rule number two.
Speakers don't require any specific amount of power. They will work with what ever amount you give them so long as you don't go so high that you exceede their limits.

Rule number tweleve
Don't use two different amps to run two fo the same sub. Also don;t run two different types of subs, it's just asking for poor audio reproduction.

Well, what do you think the weak point in the sound of your car currently is?
Improve that.

And you cannot hook up 2 amps to one sub. It doesn't work like that, and you will fry something. (There are a very few amps that you can do this with, but you dont' have them)

You didn't meantion any speakers...just subs. Do you have speakers? Do you like them? Are they amplified?

If you're only looking to improve the bass...Scrap the amp and get something more powerfull. Look for a used JBL BP1200.1 on ebay for about $200. Then look for some used Image Dynamic IDQ's in a sealed box and you'll be set for SQ and SPL.

I replaced the entire OEM speakers w/ a set of Alpine Type R speakers and added aftermarket Alpine MRV-F345 amp. It sounds a lot better than the junk OEM in the 06 Odyssey. Honda is always very weak with their sound system. The music and DVD movie comes to live w/ the new set of speakers and amp. It is worth it to bite the bullet to get the entire aftermarket component.

I recommend that you visit the show room and listen to the aftermarket system before you purchase them. Good luck.

Alpine MRV-F345:

Almost all stock car stereos suck, from my experience. Poor sound balance, poor speaker placement and poor sound staging are the hallmarks of many stock audio systems I heard, all the way from Hyundais to the Lincolns and the Audis. If I get to listen to the Dynaudio speakers in Volvos, that would round up pretty much all the stock audio options from the cheap to "the premium". It has been underwhelming to say the least, so far.

The only way to get better sound in a car is to go the aftermarket way. As previously said here, start with your front speakers - get a 3-way component speaker set with separate tweeter, midrange (vocals) and midbass speakers, and then add a decent sub. Even with moderate power input, your car setup would sound much better than most "Bose" premium car audio systems offered as upgrades in the cars.

My Toyota Matrix has a pretty good stock stereo in it. It has a nice dynamic range, an EQ, a digital program display, rubberized knobs that make it easy to manipulate, and loud enough for a casual listener. No, it cant match most aftermarkets, but exceeded my expectations.