Transl,
If your first experience with the dealer doing parts upgrades was good, why would you doubt his ability to do another upgrade to the same speakers? Did the first experience not earn your trust? Why do you feel the need to be reassured by a bunch of people you have never met or done business with when you seem to have already built a trust relationship with this dealer? If your dealer says the upgrades can be done and in his OPINION they will produce good results and you trust him, then go for it.
If your first experience with the dealer doing parts upgrades was good, why would you doubt his ability to do another upgrade to the same speakers? Did the first experience not earn your trust? Why do you feel the need to be reassured by a bunch of people you have never met or done business with when you seem to have already built a trust relationship with this dealer? If your dealer says the upgrades can be done and in his OPINION they will produce good results and you trust him, then go for it.