I have a 5 channel Hifonics amp 50x4 at 4 1 x 150 at 4
And I have a JL 10" W0
This provides OK bass...I like the sub, its nice and tight but I want it to be a little more loud. Hit a bit harder....I don't want a whole lot more maybe 50% more than what I have. Now my question is do I need another 10" sub (in which case the amp might not be upto it). Or should I get a 12" and a mono amp with more juice?
you could go with a JL 12W0 which is rated for 125W so it'd be a good match or look at two Elemental Design 12" subs on an amp like a JBL or Kicker monoblock. depends on budget and how much harder you want them to hit. the twelves will be an improvement. going from one 12" to two 12" will add +3dB +10dB is "twice as loud" which requires a tenfold increase in power.
adding a second sub, or adding twice the power will yield a +3dB gain. both together are a +6dB gain. going from a sealed to a ported box also yields a +3dB gain.