Dave reeves who designed and started hiwatt was contracted to design amps for sound City. As far as I know he designed the mark 1 and 2 sound city's and left before or during Mark 3. Mark 1-3 are a similar circuit with revisions and are very close to a hiwatt. Then the mark 4 amps that came after are totally different.
These have partridge transformers and a nice build quality (but not hiwatt neat). They often get criticised as being cheap builds but they're as good as most vintage builds I've seen like laney, Marshall, Carlsbro, Simms watts, matamp....:it's just hiwatt is next level neat and are perfect.
Can find more info here http://soundcitysite.com
There's a history section