I would think they are not authorized. I would guess Jens but I would be very surprised. I can tell you if you have your Diezel modded by a non authorized guy your warranty will be toast.
Can a Herbert MKI be updated by a third party to MKII spec? I thought things like the transformers were different, which would seem difficult to get unless you're an authorized service center.
BTW, Jens has started his new amp service business in L.A. and is working with his father, a retired electronics engineer, repairing and modding amps of all brands. They work with all needed testing equipment like tube testers, audio generator, oscillosope etc.
I brought 4 of my amps (Fenders + Marshalls) to him lately and he has done such a fine job with all of them and they sound better than ever before. Since he is also a very good guitar player he can demo the amps perfectly. Turn around was super fast. Highly recommended.
The website is http://www.krusekontrol.com (I was told full blown site is coming this or next week).
They also carry tubes, incl. the EL34-BG power tubes from Ruby which I purchased.