First the published 4 ohm specs on the Hifi 1000d are overrated , the BRZ line are more accrute and overall a much better amplifier than the hifi line, but you are running a single 3 ohm W7 correct ?
Try a 2 ohm mono instead like Kicker ZX line or a Clarion XH7510 would be nice as well, or a HQ 2 channel that will bridge below 4 ohm, like a DD C3b or C6b