What sub combination is the best?

[sylonien]

Just another query. I was wondering which of the following combintion would be better (fictional):

1. 1x 10" 500Watts Sub
2. 1x 12" 500Watts Sub
3. 1x 15" 500Watts Sub
4. 2x 12" 250Watts Sub
5. 2x 15" 250Watts Sub

See where I'm getting at? Basically my question is it better to have two smaller/less powerful subs or would it be better to go for a single large/powerful subwoofer?

The reason I'm asking is 'cos I want more bass, but not sure if I should add another one to the my current JBL 12" 250W RMS/805W Peak sub or ditch it and go for something else?