Originally Posted by Archaea
Other AV supply shops all mimic the above example from what I've seen over the years. If it's a myth is perpetuated lots of places...
That's how it becomes a myth. If only source said so, it would merely be an error.
I do not have reference off-hand but my statements are based on basic electrical circuit knowledge. Devices that specify 75ohm impedance (or any other) do so because they provide a matched input/output impedance. So, your cable box or TV has a 75ohm input impedance for the RF source. Same for DACs and other digital devices with coax (S/PDIF) inputs.
The input impedance of typical subwoofers and analog amplifiers is in the 10k-200Kohm range. The source impedance for players, tuners, preamp, etc., that feed these analog devices is, typically, in the 100-1kohm range. The difference between a 75ohm cable and a 50ohm cable is inconsequential.