At max. setting Amir seems to have measured +5.5dB but that is only compared to a set level for '0dB'.
At 3 o' clock Amir actually measured -5.5dB (or close to it) and was probably 1V or so (for 0dB he might have set 1V as reference) with about 2V in (which would be +6dB referenced to 1V)
0dB attenuation is reached at the max volume in an electrical sense.
If Amir had used the max. volume setting value as reference it would simply only have shown attenuation below 0dB. It is a 'reference level' thingy.
Passives pre's CAN create gain but use (auto)transformers which also change the impedance, are BW limited and are sensitive to source and load impedance.
Attenuators (the pots, relay ones and stepped ones) ONLY attenuate.
All (especially cheap) potmeters have a resistance track and near its end this becomes a normal conductive track (where the pins can connect to).
This means the 'ends' of those volpots show no variance.
The bigger the track (think RK27) the smaller this area. The smaller the pot the relatively bigger this 'no action' area and is one of the reasons the channel imbalance happens at the max. attenuation point.
Stepped and relay attenuators do not have this.
What Blumlein says is that there is quite a lot of production spread in those cheap used small potmeters. So it is a crapshoot near the extremes. Also these cheaper pots will start to scratch after a few years (depends on several external factors).
So what Amir measured is NOT representative for the one you might be buying or someone else owns
