There are a lot more options out there then just those two. Personally I hate Seismic and I would be thinking about saving some more cash. At $1000 you have a lot more good options, and by about $1300 you could buy a single powered sub that would shred 4 of the Seismics. Buying used would also expand your options.
While you have a power amp, going passive will also likely require a crossover of some type. (There may be one built into your amp)
+1 Cheap gear is usually more expensive than you think.
I had a pair of PV-118 (same price range as the Seismic, might be better quality). Got rid of them and found a used JBL PRX618XLF. The JBL *solo* has more output than the two Peaveys did, and loafs along with no stress. There are some of the JBLs around, expect to pay about $1000 depending on shipping.
If you give a few more details about venues (how big), SPL needed, and which instruments are going through the sub, the folks here can give you solid recommendations.