The Monolith would be a solid option. You will always run into potential Amp problems regardless who you order from.
I haven't really heard too many problems recently, but there is no telling how old that Sub actually is, whether it would be concerning or not.
If you are truly considering this, I would wait to see what they offer up this weekend. We've seen some pretty great prices on their Subs and the Monolith 12" would be an excellent option, especially on sale.
I would take that over Klipsh or the RSL.