Mike and Guido:
I just saw your October posts about the "zeel" 50 Ohm connections and want to pass along the following -
I have a Coherence II preamp, which I note has a stated 50 Ohm output impedance. When I bought my darTZeel amp, Jonathan suggested that I try the 50 Ohm connection in view of the output impedance of the Coherence II. Peter Ledermann (a/k/a Soundsmith), bearing in mind the special PIN arrangement for Rowland XLR connections, reterminated a 50 ohm cable for me with a female XLR on the preamp end, of course retaining the BNC connection for the darT end. I believe it is a 30 ft. run, but it might be 25 ft. The preamp drove the amp, but I thought I might be hearing some distortion. In A/B tests with a 25 ft. run of Jena Symphony, I preferred the Jena and left it in, where it has remained ever since. There was some speculation that the XLR termination may have spoiled the party, but I was told by others that this does not make "electrical sense". It could also be the case that the stated 50 Ohm output impedance of the Coherence II is nominal (no bandwidth is specified), and that variations at other frequencies screwed things up. In any event, it didn't sound obviously wrong, but there was something subtle that was awry, which I perceived as distortion.
Conventional wisdom challenges the very idea of 50 Ohm connections, but Herve swears by it and I have heard a top-flight PD --> darT preamp --> darT amp set-up with 50 Ohm connections and it sounded great (the owner greatly preferred it to his Jena Symphony, incidentally). I do not question that 50 Ohm connections work - it just didn't work given the particulars of my Rowland experiment.
It's hard to say whether modding the preamp to include an actual BNC output would have made a difference, but given the annoyance I experience with such experiments (my preamp to amp cable run traverses an inhospitable crawl space, the area behind my equipment shelves is tight and requires a lot of contorting to make preamp connections, not to mention what would be involved in sending the preamp back to JRDG for modding), I didn't take it any further.