keep in mind that the sub is wired at 3 ohms (nominal impedence). When playing music the amp will not see exactly a 3 ohm, but will vary with music.

Also, since your amp is 4 ohms stable in bridged mode it might not like how you have the sub wired. You may have overheating issues. It's hard to say how much the amp would be putting out at 3 ohms (if it can handle it). Just because it's a lower impedence than 4 ohms (for 300 watts) doesn't necessarily mean it's going to put out more power. Amps can be designed to put out max power with a certain load on it, and won't always increase by decreasing the load (it often will, but not always).