I believe it is simply a discontinued product. The current models are the mkIIs...
I did find this on their website; it is the owners manual for the ST250, ST500, and ST1000.
https://wyred4sound.com/sites/defaul...ANUAL_v2.1.pdf
Therefore I ascertain that the ST models are now discontinued and the current stereo amplifiers that W4S offer are the ST500 mkII and the ST1000 mkII.
I have no idea how long ago the mkIIs were released but I assume that the ST250 was around shortly after W4S was formed as their own independent company and therefore was not receiving a ton of press yet
.... although it does seem that they did get pretty decent endorsements fairly early on because of the great reputation that EJ and crew had doing OEM work for PS Audio and others with Cullen Circuits.