As far as I can see that review does not address the H2O Audio S250, in either the Signature or non-Signature version.
Here are two sources indicating an 8K input impedance. Note that at the first link the 8K number is stated by Mr. Ho himself, for the Signature version. (The second link is for the non-Signature version):
https://www.audiogon.com/listings/solid-state-h2o-audio-s250-signature-analog-icepower-amp-2014-06-0...
https://www.gearogs.com/gear/83438-h2o-audio-s250
Regards,
-- Al