My apologies if this is covered often. First, what is the rule of thumb as to which tubes (between preamp and output) change which aspects of tone? Next, how does one know which tube to replace among premp and/or outputs (V1, V2. . .etc. [by the way, I don't know what 'V' even means]). My specific example: I have an amp with five 12AX7 preamp tubes and two 5881 (6L6GC) output tubes. If I wanted to take some harshness/brightness out of it, what am I looking to change? Is there a nice website that would help me gain this type of knowledge? Thanks.