This answer is valif for the circuit shown only. A different circuit may and will behave in completely different ways:
A DC voltage on the XLR connector is unlikely to damage components as long as the voltage is within the rated limits of the XLR input. The headphone output is decoupled by capacitors, so no DC will show on the headphones (DC on speakers/headphones may cause damage).
Am i able to apply a low DC voltage instead of AC to the XLR for the purposes of testing components?
When testing components the circuit shall not be powered by a power supply. Otherwise you risk damage to the meter or to components. Also, testing components while built into a circuit is always tricky as the other components and the PCB traces will tend to change the readings of your meter. For example try measuring a resistor built into a circuit - you will more often than not see a resistance that is not what the resistor is supposed to be due to other resistances in parallel. You can use that kind of measurement to due a quick first check (open or short circuit), but you'll have to de-solder at least one pin (of a 2 pin component) to get accurate measurements.
I suppose you don't have an oscilloscope which makes it rather easy to follow AC signals.
In that case I recommend for that kind of circuit (audio) you use a signal injector and tracer (Google). The injector generates a low voltage AC signal in the audible range which can be applied to different parts of the circuit. The tracer is used to pick up the injected signal from other parts of the circuit to check whether the signal path is working correctly. It takes some time to become familiar with the usage of these tools. Once you know how to handle them it allows quick tracing of signal paths without having to look at each individual component. Leave the latter for the final examination once you have pinpointed the source of the problem.
