OP
^^^^^ thisif you wire your speakers in series ( 8ohm for each speaker ) and bridge them both @ the amp... You are goin' to hear a difference!.... There's no need for you to get another amp right now if you're not going to upgrade!
Sense you have (2) dual 4ohm subwoofers... You need to do the following:
1) connect voicecoil 1 ( whichever one that you choose to be #1 ) take the positive of and connect it to voicecoil #2's negative!
2) run voicecoil #1's negative & voicecoil #2's positive out to your amp ( bridge mode )
3) do the exact same thing to subwoofer #2...
When both subwoofers are connected in bridge mode @ the amp...
... You will be at a 4ohm load!