In the case of the JBL's, they have a great return policy so it is no-risk to return and you get to try them out in your own room which is much better than in a showroom. From the JBL online store:
Return Policy
The following Return Policy applies to purchases made directly from our online store.
Free Returns Restrictions:
If you are not completely satisfied with your purchase then please contact us. We will gladly exchange your product or give you a full refund, even after the holidays. We will even pay to have the product shipped back. There is absolutely no risk to you. Gift wrap and shipping charges are not refundable. Personalized product exclusion: Once personalized orders have been processed, they cannot be cancelled or returned.
But, unless you just don't like the look, I don't think you would return them! These are competitive at MSRP, but with these sales, I can't think of anything that can begin to compete in the $700/pr to $1000/pr price range. Be sure to check the size of the 580 vs 590! If you read the thread I linked above you will understand that the 590 is BIG!
Which Advent do you have? I used to have AR 3a's and KLH 6's (both designed by Henry Kloss, who went on to start Advent and then Cambridge SoundWorks) and my brother had Advent Legacy's!
In 2009, I decided to update. The new speakers are definitely better resolved, especially in the upper half of the frequency range!