First of all I would be stunned if the 401's went to 30 Hz. 2nd: even if Mtyhoweveryouspellit is wrong, it's unlikely he's snide. I have a couple of cheap subs worth about $500 all put together and I prefer music without them because my xo option is locked in at 80Hz in the Oppo player's bass management. His points hold true in my situation.
There's a pretty strong consensus here to upgrade the mains but if the OP is satisfied then $500 on one sub would be my advice. You should order it today.
EDIT: Oh yeah, I can't stand jazz and pop isn't real big on my list either so I don't miss much not using my cheap subs.
2nd EDIT: It just occurred to me that Irv's point holds true on a different system that has two particularly sweet sounding DIY subs but I think that may be due to my listening at low spl. Way below reference level it's nice to be able to bump the gain putting the low end up past our threshold of audibility. Without the subs I'd need an EQ to get that. Different strokes and skinned cats.