For the love of Millicent S. Glandmonocles, I'll vote already.
If we understand one another, Feiao -- and I think we do -- then you've probably guessed where I lean (and, no, I don't like to be touched there).
C and D seem clearest -- not only to me, but probably to the gawker who's never even seen an amp before. Additionally, D seems most accurate, since Normal is a standard setting on traditional hi-fi equipment and, as a term, is understandable to laypeople (who apparently provide sex to the entire world when they're not staring vapidly at switches on headphone amps). It is also a relative term in the sense that it can mean the normal sound of a given amp -- even if that sound is particularly bright or dark, or boosts the midrange to some alarming degree that causes people with autism to drone dipthongs into their teacups.
Two problems with using the word Flat:
(1) You have to know EQ terminology to understand it and not every customer will.
(2) As I insinuated above while pointing and leering, no amp truly is.
And now I must return to my highly special project: Mastering the art of smiling and frowning at the same time.
Edited by scrypt - 10/21/11 at 5:58pm