You should have a legitimate question as to what dB’s the amp is referring to. What brand and model is it?

First, is it actually specified as dB’s? Most amps I’ve seen display volume on an arbitrary scale, no units specified.

Second, the amp has no way to know dB sound pressure, as that depends on the rest of signal chain.

So is the displayed figure referring to dBu, dB mV, something else, or something totally bunk?

If 0 dB is its maximum, then -50dB is 50 dB down in power from its maximum (so 10^-5). No m V or other post-fix needed.