No difference in tone with tube-pre bypass

I have a Carvin BX700 with a 12AX7 preamp tube...and I'm ignorant when it comes to tubes.
But when I engage the tube bypass - the tone doesn't sound any different.

So...is the tube pre just marketing baloney (I'm assuming this is the case),
or could it be a bad tube...
or could it be the case that bypass isn't actually working...

I looked at the tube and I can't see any visible signs of anything wrong - but it is a GrooveTubes 12AX7 which I've seen bashed here and there. So a replacement suggestion would be good (I've read that JJ's are supposed to be superior).

But if its just a gimmick from Carvin, I'll save my money.