paralleled up to give a nominal 0.8 ohm load.

two things come into mind... first, the gain/bassboost might be up way too high causing clipping which is more self-evident at higher frequencies (i actually killed my first sub that way, being the audiophile newbie that i was. i had loudness on my empeg to full, and had my amp bass boost set to max, and gain at halfway... before long, i started to get a noticible rattle, and had to get the driver replaced.).

second, i'm no real expert, but if i recall (and from my own experience) the less resistance the speaker has, the louder it gets but the quality becomes less. i would try to hook the pair in series and put the gain up to compensate and see if it helps w/ the rattling.