Here is a comparison of two high end preamps, all else being equal as close as I could make it. If you feel there is a volume difference then tweak the playback to compensate.
http://dcoombsguitar.com/Guitar%20Music/NPNGD_DMP2NW_and_GreatRiver_MP2MH.wav
These are both very nice neutral (uncolored) preamps so the difference is small but evident.
Like I have mentioned before, my gear has evolved from MAudio Omni and AKG C1000s forward to better gear. My first CD was this early gear, my second Great River and Gefell M300s. My third Great River preamp and M295s. My fourth the same plus some room treatment wiht acoustic panels. Some of my recent recordings (posted on the Show and Tell and here) a NPNG preamp, Gefell M295s and a wood floor in place of carpet. Some reverb software upgrades along the way also. All during this time I don't play any differently or mike any differently on average. You can here the changing results if you care to listen.