It is quite difficult to come up with an universal approach on the long and old story about impedance matching.
During that time of audio pioniering, the late 40 early 50s there were several ideas on how to couple different pieces of audio gear.
Historical speaking the was the 1mW into 500 Ohms that originated at the telephone companies of the time, and was changed to 1mW into 600 Ohm right after WWII.
1mW into 600 Ohms equals 0.775V or 0dB.
Outside of Europe the power coupling or impedance matching system was used, what means that the load voltage is 50% of the no load condition, were the 600 Ohm is nothing than a compromise that arose from the best telephony frequencies, but getting outside of that range you end up with mismatching and distortion.
Mostly low frequencies are saturated and so for automatically cut out of early recoding equipment standard.
Within Europe there were two different school of thought, the english one and the german, the brits had an modified version of the American standard and the germans used the voltage coupling idea that does not require any fixed impedance.
That idea became the now "to-date" standard and gives the simple idea that the frequency range of an piece of audio gear is best when no load is present. In practical application there is always some kind of load that is level and frequency dependenend but in general you use four time the output impedance of the item in question. (That is the general standard of the U and V series)
So what to do with the old junk? In case of units that were designed for impedance matching, there is almost in any cases an audibly improvement having no load resistor at all, lets say 600 Ohm output into 10kOhm, etc.
Sometimes with high level outputs (something like an RCA unit with two 6V6 tubes as the driver) you end up with overshoot as well as resonances that create funky sounds, in that case you measure no load conditions at 40, 400, 1k, 4k and 10 k Hz, than you use a potentiometer (2 k Ohm should work) and at every of those test frequencies, then you adjust the potentiometer to 50% of the no load level (if no load is 4.3V, 50% should be 2.15V), take all resistive readings and get an average for the compromised resistive load.
As an example
40Hz 335 Ohm
400Hz 450 Ohm
1k 600 Ohm
4k 548 Ohm
10k 345 Ohm
So a 460 Ohm should be fine, 1/2 a watt works and if you want to be sure get a 1 watt resistor at www.mouser.com
Please be advised that you need an audio level meter to get proper frequency readings, most of the standard mulimeters just give you an estimation on audio frequencies.
There is still a lot missing in giving a proper answer, because this is not a simple question, technical speaking the output of any audio gear was specified, inductive, capacitive or resistive, so any graphs published are nothing else than a perfect situation.