Dog, I'm still hanging on to the original advice I gave you - Sell the wife. This almost ALWAYS results in toy improvement.
Now that we've got that out of the way again, Jack has done as good a job explaining this issue as might be done. A sub is better than no sub. A good sub is better than an OK sub. A great sub is a thing of beauty that is hard to define. Your JBL falls into the "better than no sub" category. The SVS/HSU recommendation is 2 levels up. They take you from loud, muddy noise to a real enhancement to your HT. The difference is being able to hear a note start and stop on a bass guitar. An explosion is clearly definable with a start/stop, versus something that sounds like a roll of thunder. If $ are a concern, the room you have will be very well supported by a HSU VTF-2 or STF-2. A good sub may be more important than surrounds.
This is just one old guy helping another old guy (Jack) out, but you really owe it to yourself to get the sub.
