These terms are used here quite a bit and often the context is something like 'the volume is about the same but I get (or lose) the headroom.' To me, you have preamp headroom that is set by how hard the pre is being driven, and you will retain that tonal signature at any volume level within the capability of the power section. And, you have the headroom of the power amp and that will always be tied directly to volume (barring attenuators). So, you want more clean headroom, assuming a clean pre signal, you need more power and greater volume capability comes with that. But make sure the preamp gain stages have headroom for the signal level you are sending first. Is there some other interpretation of the headroom term?