I quantified the impact of tone with a valve amp above.
To clarify - if the volume coming out of the amp is lower, the less gets to the vocal mics.
if you don't run a valve amp hard enough, you may not get the speakers working as they should and they won't give the tone that you are looking for. Likewise, if you don't run a valve amp hard enough, the tone will be very bass shy. In order to get the tone right you'll need to work the amp and speakers right -and that usually results in unfriendly volumes for a sound engineer.
in terms of on stage noise rejection, mics with high off axis rejection are the obvious choice.