With your supplied data, a 4^3 ft. ported box and a 20 Hz tune I got 105 dB at 20 Hz. I don't know how great this is since I haven't finished my first real sub build. This is compared to my JL Audio 10W7 build. You're in blue i'm in yellow. My sub is pretty much exactly 6 dB higher than you from 20 Hz and up. If you build another one of the same size and power you'd gain 6 dB and would pretty much be exactly on the same curve as my one sub. As mentioned before I think to get any type of enjoyment out of it you'd have to build two.