The BNC inputs and outputs and their 50 Ohm impedance are unique to the DarTZeel I believe. I cannot think of too many other power amplifiers with a 50 Ohm input impedance. Those on the low side are usually around 5K-10K and many tube amplifiers have input impedances over 100K.
Theoretically the impedance matched BNC to BNC connection would be ideal for extremely long runs with no loss. Note Hervé is using 50 Ohms, not 75 Ohms which is usually specified for video and digital audio applications.
The BNC connection "should" be the best connection between the preamp and power amp because they are impedance matched, versus a typical 600 Ohm output impedance on the preamp driving a 100K power amp input impedance (just an example).
For greater in-depth understanding on why Hervé believes his 50 Ohm impedance matched connections are the best and only way to transmit an audio signal, please read his article in the November 2001 Stereophile (kind of technical). Also reference pages 26-28 in the "b" version amp manual, paying particular attention to sections T5.4.4 and T5.5
High quality properly constructed and terminated 50 Ohm cables are all you should need for great sound. No fancy platinum, air dielectric, etc.
Here is one 50 Ohm cable I'll be trying from L-Com- 18 awg solid silver coated copper with 2 silver coated copper braids for shielding. Wide bandwidth and extremely cheap- no loss in trying them.