That 3 dB reference is wrong...I've seen a bit around here too. I think they got that confused with the "double the power" thing...
1 dB is arguably the first noticeable change...guess that makes sense - they picked this unit for some reason. Although I have some litereature in front of me that suggest 1/2 dB is known to skew stereo imaging off-balance, so with 2 reference volumes, 1/2 dB changes are noticeable (David Weems). Geoffcin, this would certainly explain why you can notice that 2 dB bump. And if it's over a wide enough band relative a -2 dB dip in an upper band it's even more pronounced. I'm with you on that, 2 dB can make a huge difference...DIY-ers are always padding tweeters with slight differences in resistor values to get that perfect spot balance between detail and sibilance...often just "1 or 2 dB" from 10KHz -20 KHz.