Audio Advisor should allow you a 30-day money-back guarantee.
I've heard the 340, 540, 640 and 840. For me, the 640 is the sweet spot in terms of value.
However, given that Cambridge products in a certain "series" (340, 540 and 640 being one and 740 and 840 being the other here) have the same sonic flavor, the lower models always offer a generous taste of the higher ones because they tend to be based on the same basic architecture.
Therefore, if you could find a used 740 -- and depreciation is usually pretty steep on Cambridge -- that may end up being the ultimate value. I bet that sooner or later you could nab one for around $550 to $650.
I would also strongly suggest you listen to the Rega Apollo. It's not as detailed as the 840 (nowhere near, actually) but it may end up being the more musical player depending on your person taste.