I just put a 150W RMS amp to 2 10" Kenwood subs in a sealed box in my convertible. Though QUITE loud, it won't be heard a mile away. The answer to your question really depends on your goals.
Also, going too extreme will require you put in caps and possibly larger alternator. Do you want to go that route?
Another item to consider, when you install subwoofers, you may end up not turning up the volume so much anymore.... as that "missing" sound you were craving is now there.
Lastly, I've read many times that your amp RMS wattage should somewhat match your speakers' power ability i.e. a 200w signal for a sub rated 200w nominal etc.... you can underpower speakers a bit... but OVERpowering them may fry them.
150w RMS in a dual sealed box sounds GOOD.... though 15"s would be better lol