I emailed asking if a tape head input could be added. That was 10 days or so ago and I guess I got blowed off as I never got an answer. Guess they have all the business they need.
Does your R2R have separate tape head outputs or are you using the output from the regular tape deck internal preamp?
I can't speak as to why Holger hasn't responded to your email. I do know he has specific days and hours for contacting him. My experience has been very positive with him. Usually he will place some info on their website about being away for travel etc.. Try another email or call during the hours he has listed on their contact page.
My tape deck is not modified for an external preamp. Holger and I did message back and forth about modifying it but he stated that probably would not be worth it. Reasoning that changing a vintage machine would diminish it's value, I chose not to make the modifications. As it is, the SQ from my machine is very good.
I take it that you do not currently have a machine? Buying one now can be tricky. Ebay is full of people selling machines but a quick look shows that most are estate sale purchases that they know little or nothing about and operating condition is unknown. I picked mine up locally so I had a chance to see and hear it before making the purchase.