depends on your budget and at what impedence you want to run them....if you get D4's you will have 0.66ohm load(parallel-parallel) and D2's would result in 0.33ohm(parallel-parallel) or 1.33ohm load(series-parallel)
i would go with D4's (0.66ohm) and put em on a saz-2500
edit: also will need good electrical to run that power