"Variable output impedance is "heavy disadvantage" of any passive preamp even if you have substantially high input impedance of your amplifier."
Wrong Czarivey, ^you stated^ and the reason is.
A 10kohm passive can have a varying output impedance, at it's highest (worst) it's 2.5kohm at mid position.
Any amp/s that have input impedance 47kohm and especially higher will have absolutely no ill effect on the sound with 47kohm it's close to 1:20 impedance ratio. And if 100kohm input impedance the ratio is then 1:40 ratio. And if the volume is lower or higher than mid point, the ratio/s are even higher, still with out any effect.
The only thing that needs to be watched is interconnect cable capacitance that can cause a HF filter.
The HF filter caused by the interconnect capacitance and this high 2.5kohm output impedance. (BTW: same goes for many tube preamps as well as they can be that high or even higher)
If we look at a bad quality interconnect cable that has 200pf per foot of capacitance, that equates to say 600pf for a meter. This combined with the 2.5kohm of the passive pot will give you a -3db at 106khz! Well beyond our hearing.
All good quality interconnect that I have measured are below 100pf per foot, and this equates to -3db at 212khz! Up in bat hearing territory.
There's just some of the math on this without any voodoo.
I ask once again show the math without the voodoo to counter this if you can?
Cheers George