Hattor Audio preamps?

Can someone give me the rundown again as to the rule of impedance compatibility?

Thanks!

the higher the input impedance of the power amplifier, the better, the minimum from which I would probably be interested in passives is 50k

I've used a Khozmo volume control/ "preamp" for several years now in two different iterations, and I would be hard pressed to go back to an active preamp despite having owned a few excellent ones through the years.  Sonically, it's about as transparent as you can get and is the embodiment of simplicity.  Build quality is top notch, and customer service from Arak (despite being in Poland) has been exemplary.  My highest recommendation.

I do not own the Hattor but own Arek's Khozmo passive. Excellent passive but definitely prefer the Dave Slagle autofomers!

I purchased a Khozmo preamp a few months ago.  I had been using a 12AU7 tube buffer.  I did not like the volume control.  Not linear but sound was good.  I bought the Khozmo to keep the cost down(vs Hattor)  I have upgraded resistors.  The build quality is superb.  Aesthetically, this is a unique and nice looking piece.  Sound is transparent.  Their equipment is customizable with the intent of unaltered sound.

Arek is very easy to deal with. I emailed Arek and hoped everything was OK in Poland.  He is housing a Ukrainian mom and her child.  What a noble gesture.