We know for RCA input into a headphone amp, the signal will go through pre-amp first and then the power amp, and finally into the headphone.
My question is if I use the RCA input and RCA output of a headphone amp, am I only using the pre-amp part of the headphone amp, or am I also using the power amp part?
There is no way to answer this without looking at the PCB of the amplifier in question.
Headphone amps don’t have preamps (basically a buffer and driver stage), they use a simple potentiometer to reduce the signal strength. Then comes the amplification stage that has its own input and output stages.
The ones that can serve as a preamp are simply taking the signal from the back side of the potentiometer and outputting it. This is not a traditional preamplifier with a buffer and driver stage. Although it may be considered a passive buffer (I’m not sure on this point).