Do you have both subs wired in parallel, bridged across the 2 channels?
If so, I think that will give you 2ohm mono. The instruction manual only quotes power for 2 ohm stereo or 4 ohm mono, so I thought the amp could only accept a single 4 ohm speaker when bridged. But hey, if it's working okay for you, I'll look into doing the same!
My sub is currently in a quite small (0.75 cu ft) sealed box, which could explain my lack of bass.
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black