system seems to be lacking much

Hey guys, I am new here to the group, and I am looking for a little help.  I have read most of the posts on here and have found alot of useful info, so I signed up.

My system (created on a very tight budget) seems to be lacking much.  I have a Pioneer 760 watt amp powering two old 'ground zero' subs that I know nearly nothing about.  I had 4 12's awhile back (Rockford HE2's and 2 JL's) but was forced to sell them when I needed cash.  I got these junk subs for a partial trade on the HE's and they seem alright, but they really don't hit like they should, or even like they used to.  I hooked them up and adjusted the amp up until I had it all right, but they aren't as loud as they should be.  Everybody says my amp should be drawing much more power than it is because my lights don't even dim, but it seems that I can't make these two subs sound good, even though they are in decent shape.  Where do I start in making it any louder?  How do I get that rumble back?

They are in an old sealed box I had made a long time ago.  I'd say the box is roughly (LxWxH) 16"x36"x16"

They are wired one to each channel, being that they are SVC, just POS to POS, NEG to NEG.

The amp is weird, it has no frequency adj., and I don't know how it is set...  Will check when it is light out tomorrow and post back.  I remember it was halfways low, sounded best with headunit power down and amp boost on +12...

I am going to upgrade alot this summer, but having a half-... system is just annoying to me. 

Thanks for the reply!

Ok, so one sub is pushing air into the box and the other is pushing out.  It is loudest that way, by far, and I get the cleanest sound.  I have heard that since the box is semi-small, I could port it and make it sound better.  I was thinking about doing it but wanted input first.  If I ran them both hitting out together with a port, would it sound better?

My buddy has 4 twelves and 2 amps in his 2000 Blazer.  He has them in a custom built slot-ported box that his brother and I built last year.  We put alot of time, figuring, and labor into, and he finally put it to use this spring and it pounds.  Is slot porting the way to build a box if you have the room, being that they require a little more air?

I am really sorry, I know I sound like a person that has no idea about a system, but I have had a decent one for years...  I don't know how to operate with what I have now though...  Thanks for all the help!