There are very few systems that do not benefit from subwoofers and bass management. However, adding cheap subs without a dedicated 2 way crossover is frequently worse than having no subs. To add subs correctly you either need a room control system with bass management or a calibrated mic measurement system like Dayton's Omnimic along with EQ capability. Trying to do it by ear is very difficult if not impossible.
Doing the DIY thing is a good way to get higher quality subs at a better price and there are some very capable plate amps available. I personally prefer passive subs with outboard amps and crossovers. I would never go less than two 12" subs or four 10" subs. Contrary to lay instinct, larger drivers are not "slower" than small drivers and they have significantly less distortion. Get the largest ones you have room for.
Doing the DIY thing is a good way to get higher quality subs at a better price and there are some very capable plate amps available. I personally prefer passive subs with outboard amps and crossovers. I would never go less than two 12" subs or four 10" subs. Contrary to lay instinct, larger drivers are not "slower" than small drivers and they have significantly less distortion. Get the largest ones you have room for.