i think personally this is the most open ended question to ask. No Offense but there are soo many subs amps and boxes out there someone with the best equipment can make their system sound awfull because of a poorly designed box. I would say the best thing you could do is set a budget for yourself on each item and go from there. I have heard 8's 10's 12's 15's and 18's in every application they sound different. so i guess its a starting point if your trying to accomplish sq or spl. Sq run a sealed box spl run a huge ported box. I dont wanna reccomend subs soo many good ones but here are a few brands to check out digital designs, re audio, image dynamics, kicker, phoenix gold, jl, all good in different aspects. there are many more killer subs out there too