Interesting ethics discussion. Assuming no patent laws are knowingly broken, I have mixed feelings on this. Thinking out loud:
- If it's an exact clone of an in production model (that is actually available to buy***), made and bought to be cheaper than the original, then that doesn't feel right.
- If it's an exact clone of an in production model self-made for the hobby of audio electronics DIY and reverse engineering, then that's ok in my eyes, so long as it doesn't get used in a professional facility in place of the original.
- If it's not an exact clone, but a purposefully modified one that seeks to change/enhance sound or functionality, making it a variation that isn't and can't be offered by the original manufacturer or achieved with an original unit... then there's some ethic legitimacy to that endavour imo.
In any case, I think none of these "clones", regardless of how faithfully recreated they are, should be labelled or referred to by the SONTEC (or SSL, etc) brand names. Not even as SONTEC clone (or SSL clone). Using the name on a front panel or company web site imo seeks to gain an advantage from another company's reputation and is hard to justify ethically.
*** Some Sontec models, for example, are very hard to come by