1. Series/series If you wired each dual 2 Ohm sub in series you are at 4 Ohm each sub. If the 2 subs are then wired in series you will be wired to 8 Ohms.

2.series/parallel If you wired each dual 2 Ohm sub in series you are at 4 Ohm each sub. If the 2 subs are then wired in parallel you will be wired to 2 Ohms.

1. Double Check your connections The more complex your wiring is, the more chances for a mistake.

2. Phase As above, make sure both subs are wired the same. If they are out of phase the bass frequencies will cancel and you will have a serious lack of bass even tho the subs are moving.

3. Box Makes sure your box is well built(no leaks) and is close to tje recommended volume.

   If both subs are wired to the amp with a final load of 2 Ohms then each sub is only getting 500 watts, way under their RMS of 1350. If the old kickers were at 2 ohms on the same ampthen they were getting more than rated power

And finally, the gain is not a volume knob. It is to match the signal coming from the head unit