Hi Michael. I really don't know anything about this brand but I do have a few thoughts.
I don't think there is any connection between current Multitone amplifiers produced by the Dutch company Koch and the 1960's Multitone amps used by the Dutch band The Sound Magics. If you read the About page on the Koch website they say "Koch Guitar Electronics has been designing, building and innovating guitar amplifiers since 1988".
So, where would a mid 1960's Dutch band get their amplifiers? My first thought is the UK.
There still exists a Multitone Electronics Company in the UK that began business in 1931. Through the years, they have been involved in producing a variety of electronics products from the first hospital paging system in 1956 to hearing aids today. While I can't find any evidence that they ever built instrument amplifiers, they have clearly been in the amplification business for a long time.
My best evidence that they may have produced the Multitone amps comes from their logo. Compare the logo on this old Multitone tube radio to the logo on the amplifiers used by The Sound Magics. Both logos use block letters and all capitals:
![Image](http://www.vintaxe.com/forum_pics/Multitone_amps_1966l.jpg)
![Image](http://www.vintaxe.com/forum_pics/Radio-Multitone.jpg)
While this similarity certainly doesn't prove anything, it is suggestive that Multitone may have dabbled in musical instrument amplification during the mid 1960's electric instrument boom.
Hope these thoughts help you along with your research.