Commercial Audio Attenuators
Modern audio is generally voltage transfer not power transfer, so sources are assumed to be low impedance and loads high impedance, typically for line level a competent source will be a hundred ohms (Pro audio) to maybe a k ohm (Unbal domestic junk), and loads are usually 10K or there about. 

For mic levels you can generally roll with 150 ohm source and maybe 2k or so for the load, but remember that your pad needs to pass phantom in a sane way.

One thing you will find is that for anything more then about 10dB or so you can probably find a set of resistors where the exact source and load make very little difference, for example an L pad for 10dB could be something like 3.3K series and 1K shunt, a few hundred ohms of source impedance makes little difference to that as does swinging the load from say 10K to 1Meg...
By the time you are doing 20dB (10K series, 1K shunt) pretty much any sane source and load Z will make only a fraction of a dB of difference, and that is what the volume knob is for.

600 ohms has not been a major thing in audio in the last 40 odd years, it was only ever an import from the phone companies who deal with long enough lines for it to matter.

Incidentally, Canford are cheaper then Rothwell...