nearly all these hubs are built in one of just a few ways; the end caps will either pull off or unscrew (even a view of the spare parts diagram will tell you which), and the bearings will sit up against shoulders on the axle, or there will be a spacer tube between them, or (infrequently) there will be some kind of shim arrangement with the end caps acting as shoulders.
[ A further (but also rare) setup is that there is a cone and locknut arrangement so that the bearings (which are a sliding fit on the axle or on the bearing carrier 'cones' in this case) can be adjusted when they develop a little free play. Maxicar and some Campag hubs are like this.]
Tune hubs have a reasonable website and there are manuals on there which you should look at if you have not already;
http://www.tune.de/en/manualsBTW it is not always obvious that there is a hex-key fitting in either the end caps or the axle. Sometimes there is both, and when you insert the key it engages with both the end cap and the axle. Needless to say if this happens the end cap won't unscrew!
cheers