I'm getting 2 Dual Voice Coil 10" JBL 250-watt subs with a 520-Watt 4-channel JBL amp.
[For $310 bucks]

Should I be good with that?
Or should I get a new head?

 

There are a couple issues with that setup.

1. If the subs are 4 ohm DVC, that amp will NOT work for those subs. (It would drop the resistance too much for the amp to be bridged on the 4 channels)
2. That is not a bad deal for $310 but you could do better.
3. A new headunit is always a good idea. If the source is better, the output is better.


Here is a setup that would blow that out of the water for $300.

Amp: http://ikesound.com/product-product_id/2214($115)
Subs: 2 of these, http://www.edesignaudio.com/edv2/pro...products_id=30DVC 4 ohm. ($180)

Thats $15 cheaper and would blow the JBL stuff out of the water!

 

PReals suggested setup will perform great. I myself would upgrade the headunit first. If $300 is your budget, you can get a great performing headunit foe that. You will see significant performance gains just simply by uprgrading your headunit. If you want the bass that bad, do it. But I suggest headunit is the first step.