Design changes introduce subtle (and not so subtle) differences along the way, and these differences are cumulative over time as more changes are introduced.
There is a reason IK Multimedia has sought out classic amps from the "right" years. Not just any AC30 would do, for example. But when it comes to Hiwatts, they are made by hand exactly as they were 45 years ago. IK could use a new Custom 100 or a vintage one, and the only difference would be the tarnish.
And I bet they could even model that.
