If you know that the source will always be a vaguely modern output like an op amp, which is usually at most 100R (and not another filter or some high Z tube out), you really could just add a series resistor to the input terminals. Maybe create a cable even if it's just a terminal strip. Meaning you could add 500R but in practice, the source resistance doesn't really need to be exactly 600. It can be quite a bit less so you could do like a 220R with the assumed 100R or maybe even a 100R. It should not really have an impact on the response. And, if the circuit is a typical filter of 600R characteristic impedance, you will have much less insertion loss. Meaning make up gain becomes less important (but it might be enough that if you want to switch the thing in and out at the bay and have the level be exactly the same, you might need a make-up device).
If you know that the destination will always be a vaguely modern intput, which is almost always 10K, you can just strap a 680R resistor across the output terminals or integrate it into a clearly marked cable if it's nto a terminal strip. This load does need to be near 600. So 10K || 639R makes 600R. But the next standard value up of 680R makes 637R which is just fine.
You should definitely break out the spectrum analyzer software for this though. Run white noise through and look for loss and peaking. If you have loss at the high end, increase the load resistor value to 1K and see what happens. If you get peaking decrease the value to load more. But try it with different sources.
The only caveat with all of this is if you want to patch one 600R passive filter into another. Then it won't work. You might think that wouldn't happen much but unfortunately I find that I sometimes do have some high impedance device of some sort that really needs to be buffered. But there are some options.
One option is to do the impedance matching at the patchbay. More specifically, and I'm just thinking out loud here - I've never done this before, instead of having one pair of jacks on the bay for the device (let's say you use upper for output and lower for input) you have two pair for the same device right next to each other but the left pair is labeled "NO DAMP" and the right pair is labeled "DAMPED". So on the right input you have a 220R in series with the + signal wire and on the right output you have a 680R load across the + and - signal wires (note that the NO DAMP output will have to be normalled to break the load of the DAMPED output). Now you can plug into any combination and get the right damping. For example, lets say the source is another filter, you can patch into the NO DAMP input. But the destination is a modern 10K so you patch from the DAMPED output. And so on.
The other option is to build a buffer device that is just a little extruded aluminum enclosure with cable gland and some wires coming out of it and you tuck that behind the rack. It two buffers each with 10K inputs and 100R impedance balanced outputs but of course with send / returns that are damped properly. Total two dual op amps. Use a 24V external SMPS and make the virtual ground with a voltage divider and a few transistors for +-12V. Something like that.
