With out more details it is a little tough to make a recommendation. The speakers you have look to be a little iconic themselves, so you may inadvertently buy something that ends up being more of a downgrade then an upgrade. Usually when people shop for used they have a very specific model in mind that they are familiar with and watch for.
There is a lot of risk purchasing used based on someone else's recommendation, because they just might not be what you are looking for... but you will be stuck with it. (Unless you know what you are buying is such a good deal that you can re-sale it for more.)
That being said check out Vandersteen's Model 2ce and newer. Just watch for a while and you can usually find a good deal for a pair of them between 400 and 1000, I wouldn't pay more then 600 myself though.