Yes, another subwoofer will work.
And, if you feel these surrounds are worth $400/pair. then go for it.
Yes. they are JBL, but the 212 sub/satellite was recognized as one of their marketing miscalculations. Not that that makes them junk but if you're looling for a "classic" JBL sound, then this will not be it.
If you're looking for a large, bass shy pair of speakers that have the JBL logo, then you've got it.
To put it simply, you can get better sound for that $400, even with the JBL logo. I picked up a pair of L-26's for $150 a while back.