Spendor s3/5 upgrade recommendation?

You might try the Spendor group on Yahoo. See:
http://groups.yahoo.com/search?query=spendor

However, speaking in general terms, Spendors are thought of as a very well-thought out design with very appropriate component selection. One typically doesn't see much in the way of mods being recommended or suggested.

Per Mlsstl's sentiment above , Spendor itself modded (changed the crossover) for the newer S 3/5 SE version and some (myself included) prefer the earlier version to the newer. Change is newer but not always better!

I too would strongly advise against altering the speakers. Depending on what you think you are missing, add a subwoofer or move to a different model.