With low tunings, you're going to struggle to get decent bottom end out of 8" drivers. I'd at least look at a ported 12" or 210 configuration like the Peavey Max 150 (12" - $350) or Orange Crush Bass 50 (12" - $300).
You could also keep your eyes on the used market. I'm hoping to pick up an Ampeg 210 combo for under $200 tomorrow. The biggest thing is to research how it handles the low B string. I've played through many cabinets, including an SWR 410 that just fart out on the low B string (I'm a 5-string player, but you're tuned to a 5 string's bottom end anyways).