If they're SVC subs with 4 ohm coils wired in parallel, the impedence load is 2 ohms, not 8. That amp is stable down to 4 ohms bridged so if you have them wired for a 2 ohm load, watch for that amp going into protection mode.
Putting random subs into prefab boxes not built to spec for the subs is the source of the sound quality problem. Bandpass specs are a lot less tolerant than sealed and vented enclosures. Like all other enclosure designs, they should be designed around the specs of the sub and the power given. Using some sub with unknown specs in an enclosure with unknown specs is just gambling that it'll sound good.