i've been thinking about saving myself some money by replacing the stock 12AX7 tube inside my $80 preamp with a better tube. (i think i ran across the bnb tubes site in one of these forums). is this something slightly common? would it be worth it? would i get myself a more pleasing sound or a bad electrical shock? (note: i did find a similar question on an ART pre from awhile back, but any new input would be appreciated.) thanks in advance.