Great suggestion Wayne. It ended up being a reversed polarity on the JBL. I guess mine are backwards compared to what the standard is.
It sounds really nice. A very full and warm sound. It didn't make my M&Ks sound like cheap speakers by comparison, but it is definitely an upgrade!
Not a super flat response curve, but I thought it sounded great. Maybe it's because it was on the floor (compared to the M&K which was sitting on top of it). With better placement and some EQ I bet it will measure out well too.