If you purchased one and wanted it spiffed up, maybe slightly tweeked -- as in changing out some stones, who would you send it to? Will some of the PS vendors do that?
I'd send it to Singlestone. They specialize in antiques and have a very good bench. There are probably a few other vendors who could do the work though.
I recommend Singlestone as well, with one caveat. Ari is REALLY EXPENSIVE. I've gotten quotes from him and had Lang's in SF beat them without breaking a sweat.
I would ask JBEG if they will do repairs. If they won't ask them to recommend some place other than SInglestone... that way you can get multiple quotes and compare.
I'd love to use Ari, but every time I've queried his prices have been way to high. So I've either not repaired the item (I have one sitting in my jewelry box taunting me) or gotten it done elsewhere.