"
Should be relative to a microvolt into a 600 Ohm load.

Forget microVolt!
0 dBu is steady AF established at 775 milliVolts line level -- regardless
of load Z.

It is an easy mistake to make, because dB relative to a microvolt is often
written as dBu rather than
using the Greek letter 'mu'; a limitation of the ASCII character set!!

Jeff