Not sure what your budget is like, but if you would like 2 10" subs with a 1000W amp, I have a full set up in the box with a 2 farad capacitor I am not putting back in my Runner.
I would be more than willing to sell it to ya for cheap. I understand if you want something new though.
Yes, you can swap an aftermarket sub in there, but I also recommend a separate amp to power it. This is what I did using jl stuff. Link with tons of info is in my sig.
Just wondering if a 10" would fit the stock enclosure if it is cut bigger?
It's not just the opening but the depth.
I think I'm going to buy stock in a hearing aid company.