good threaad idea imho - ususally you only find service-related posts on the internet by people who are NOT satisfied (which arent many in Diezels case anyway), so i approve of the "counterweight"!
to contribute my own experience:
Due to my lack of knowledge, when it was time for my first retube, i bought JJ E34L (and i will NEVER buy any JJ tube in my life again) - and what happened? on the next gig, right during the soundcheck, 1 pair burned through, and made the fuse explode. So, there was this little piece of epxloded fuse sticking in the fuse holder, and there was no way of getting it out.
I just wrote an Email to Peter, and had a reply the next day - and 3 days later i received a parcel with not only the complete fuse holders (the WHOLE things, not only the plastic part you screw out), but also a set of new tube retainers - just because i mentioned that i will go the 6550 route, Peter also sent me these retainers as the 6550 do not fit in the (previously used) "bear-trap" retainers.
you see? 1 Email, and 3 days later EVERYTHING is fine again. try that at any other company.
oh, and i'm an absolute "nobody" int he music scene, i'm neither famous, nor a really good player.