subs?????
 

Do you guys think it's a good or bad idea to install one 15 and one 12 inch fosgate p3 sub in my expo I currently am running 1 12 inch with mono 1200 d amp but not hitting low low thinking of adding a 15 bad choice or not or should I just drop 2 15s in

Not a good idea....most of the time you will just see cancellation and it will actually be quieter than the single 12 setup was. Mixing sub sizes is never a good idea.

Your simplest and cheapest option is to put the same woofer you're currently using (single 12) in a better enclosure which is tuned lower if you want more low end out of it. This will cost you next to nothing if you have basic woodworking skills and will make all the difference in the world. Even if you have to pay someone to do it, youll still be in it for much less than buying a new woofer.

If you were to upgrade to 2 15s, then you would need to upgrade your amp by either getting a 2nd one thats the same as yours now or one larger amp to power both woofers. Then with the added power you need to make sure you have the electrical system to back that up....which can get pricey. Then once all that is taken care of, you're in the same predicament that you're already in...you need a good enclosure thats tuned to properly play the low bass that you're looking for. Providing that you have enough space for a proper ported enclosure for 2 15s in your vehicle.

why upgrade the amp? 2 x 15's in a good box off 1200 watts wood be pretty nice wouldn't it?.

That all depends on your particular taste. And which 15s you're speaking of.

If you got some really high end 15s and ran 1200 rms to them then you really wouldnt see much output because they require more power.