[quote name='wateroftyne' timestamp='1423995553' post='2691200']
[url="http://www.bassmatters.nl/en"]Bassmatters[/url] in Njimegen?
Dunno what the trip there from Amsterdam will be like - just hoying it in the mix.
[/quote]
THIS! +1000! Paul who runs Bassmatters is THE man for bass in the Netherlands. Nijmegen is about an hour's drive south from Amsterdam (or take the train, if you won't have a car there).
I spent 10 years living in Amsterdam and while Dijkmann and Dirk Witte are okay for odds and sods - cables and the like - if you really want to visit a shop purely dedicated to bass, it has to be Bassmatters.
However, if you want to buy second-hand, then I recommend www.marktplaats.nl - the Dutch equivalent of Gumtree/eBay/whatever. You can buy pretty much anything on Marktplaats.