In reply to:

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Are they getting 390w (ish) from the amp EACH or BETWEEN them (i.e. 195w each).

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Well, they are sharing the full output power of the amp between them, each getting half of it. But was the full power of the amp specified at 4 or 2 ohm load? You *probably* get more power out of it into a 2 ohm load than into a 4 ohm load - at a greater risk of blowing the amp or getting clipping/distortion.

Car amps are usually voltage-bound, with a fair bit of current reserve, so they do give out more power into a 2-ohm load - *iff* they can take it! Is your amp specified to work on 2 ohms? 2 ohms is awfully low...

I don't think you are underpowering your speaker. But how loud do you play? How much reserve do you seem to have? (How much louder *can* you play compared to how loud you normally play)