The Sound Signature of a Headphone Amp

Hi,

I’ve been looking at headphone amps and I’ve been getting advice to buy units at wildly different price points. I own an ASR Emitter 2 Exclusive amplifier and the sound signature of this amplifier is exceptional. I was told to run cables from the tape outs on my ASR into the headphone amp, which is no problem. So I’ve looked around at Cayin, Schiit, etc.. From my inquiry with the Schiit store in CA., it was confirmed that the signal that I would hear would primarily be that of my ASR power amp but that the headphone amp still has a footprint on the overall sound so the better the headphone amp, the better the results. What know one has been able to explain is how much of the signal comes from the power amp and what percentage comes from the headphone amp? Is there a headphone amp that gets out of the way totally, acting as just a headphone input with a volume control? Any clarification on this would be greatly helpful. Thanks!