Yes, it is possible. But if I'm missing something here, wouldn't it just make more sense to plug your source directly into your headphone amp and bypass the preamp? I suppose if you have multiple sources and want to toggle between them, it could make sense.
If your RCA outs have variable volume control, when you do feed your preamp into your headphone amp, set your headphone volume between 75%-100% and let your preamp volume pot drive the volume. If you don't do this, you'll get weird voltage mismatches and can induce clipping.
If you do not have variable volume control on your RCA outs, the signal will be the same and you can control the volume on your headphone amp directly.
