REL Storm is a good choice for the Spicas. However, a while back I used two Mirage/Energy 8' subs with great results. These subs were relatively cheap at 500.00 per the pair retail.
A good tube amp (Conrad Johnson, ARC) would improve the sound of the Spicas, but not necessarily the bass response.
A good tube amp (Conrad Johnson, ARC) would improve the sound of the Spicas, but not necessarily the bass response.