If you replace the sub you are going to want to use an aftermarket amp. The stock amp will not move an aftermarket sub to make an ounce of improvement, infact you may lose volume.
So, install an amp that will accept High Level Inputs, tap into the stock wire in the trunk and then try driving the stock sub first. That may give you what you are looking for.
The stock sub in our odyssey is disapointing at best. I think it has the same power going to it as the full range speakers, just with the mids and lowes crossed out.
Jay