That's cheap multimeters. Certainly people test signals outside of mains frequencies, when using multimeters in a lab or something more serious. But if in doubt of your multimeter's capabilities, use 60 Hz if you can, because even the cheapest ones should be fine with that. It's the most basic expectation and usage for a user.
If you're referencing O2 and Clip+ crosstalk measurements, note the load impedance, as it can well be different for different loads. The crosstalk also may vary with frequency. For that matter, -50 dB is undesirable but not a lot at all (would be considered super way overkill for most speakers playback, for sure), particularly if most of the sounds in a recording are amplitude panned so most sounds bleed L/R much much more than -50 dB and the extra amount from the hardware would be trivial. Again, be careful of quoting certain specific measurements as the explanation of perceived differences.
You can make an ABX box if you really want. There could be some that are sold out there too.