Yes, the best idea - if possible - is to 'borrow' a pair from the dealer and listen to them at your place in a comparison against your Polk speakers.
Where are you located?
The Polk LSiM 703 speakers are frequently on sale for $600 or $750/pair, instead of their list price of $1500/pair. They might be as good as, if not better than, the SVS Ultras. But I haven't heard either one, so I don't have an opinion on either one.