I am using Clear Audio Aurum Beta which is 2.5mv output and my volume is on 9'oclock. If grado's out put is 1.5mv. Does it mean I have to move the volume up around 12 o'clock?My guess would be more like 10:30 or 11 o'clock. The difference between 2.5 and 1.5 mv is only about 4.4 db. There may or may not be an additional 3 db of difference because different test standards exist, as discussed in this thread.
I believe that volume control rotation from 9 to 12 o'clock typically corresponds to a volume change in the area of roughly 12 to 14 db or so.
Regards,
-- Al