Really? wasnt Sennheiser pairing with Lehmann in the early promotion of the HD800? Arent magazines, including many british like the (now american) hifi+, complying with the Lehmann BCL/HD800 combo suggested by Sennheiser?
The first showing of the HD 800 was with the Benchmark DAC1. The UK showings have been with the Grace m902 (and this has been mentioned in several UK magazines); Germany have used the Lehmann.
Hi-Fi+ mention both the Grace and Lehmann, but the reviewer did not understand the Grace and did not switch on the boost that is in there for higher impedance headphones (he said the Grace was excellent, but did not go quiet loud enough). Again, both are mentioned in the new review in the October issue of Hi-Fi Choice.
Sennheiser UK use the Grace because I use the Grace m902B myself and think it's a great amp.. Not because we tried the HD 800 with every amp. on the market to find out which it works best with.
Sennheiser in Germany are more likely to demonstrate with a German amp., rather than an American one.
No, Sennheiser do not recommend any specific amplifier to use, you are free to choose what you like. Ask me personally and I will tell you I love the Grace m902B, but that is what *I* like and there are loads of excellent amplifiers I have not heard at all and have had no opportunity to try.