Isobarick loading was the first pair of subs I ever used back around the early 80's. I understand the excitement of massive bass; it's visceral. But the Yes recordings were never known to have very wide frequency range. I would imagine that is not any where close to 30 hz that is being replayed. Have you checked the JBL specs for the 123; thats the 12 inch driver right? I think that rolls off pretty high up there. So now you are getting good bass using the sub-woofers.