I knew Tim as he exhibited at most of the shows I exhibited at during the early 1990's and up to 2016 when I last exhibited at a show. I knew exactly what Tim did with his specials including the Troughline and Revox G36 tape machine, among other devices. I also offered exactly the same service on the Troughlines. We would never supply or fit a stereo decoder to a 40-60 year old tuner that was fitted with all original parts. For a start you can't offer any sort of guarantee unless the tuner is fully serviced before fitting the decoder. This is nothing new. It's what most companies did if they were offering an "add on" to a vintage device.
Re the Leak TL12+: The problem is not with the input impedance. It is the high sensitivity of the first stage and the amplifiers overall loop gain, so don't confuse input impedance with high sensitivity. They are two different things.
In fact the original decoder was offered as a kit, because Tim did not want to spend more time with the Leaks etc. But he soon saw that it was not good as it resulted in bad application by users. It was like a Heathkit approach that was popular for decades but Heathkit put an enormous amount of effort into it. He eventually decided to stop when it was not done well by people. The units where he did it all are excellent though, and he did them in the way I described earlier.
Yes on the TL12+ basically what Tim did was a 10:1 divider to attenuate the input sensitivity. (I have to look up the schematics as it may have been a 9:1). I used the term impedance too loosly .... short cuts of a hobbyist.