best designed and constructed technically, personal sound preferences aside
What I get from this is something like, 'tube stuff that is of sound design, built well, meant to last and sounds good at the same time'.
I can think of a few that sound good but the construction is terrible (IMO of course). I can also think of a few that are built rather well, but don't bring home the bacon in the sound department.
I've always been the kind to want to know what the innards look like. I worked as a service technician during college and for years afterwards and what it looked like inside/how serviceable it is has always been a huge part of why I buy something.
I've seen stuff assembled with hot melt glue that would turn into a fire hazard if UPS dropped it in shipping. The crazy thing about that is the builder was probably trying to save a buck but actual terminal strips to hold things in place are a lot cheaper than shipping the unit back for repair! OTOH I've seen stuff were it was obvious that the designer had built the entire thing by hand, doing a custom layout to accommodate surplus parts.
I am of the opinion that well-laid out circuits also sound better. This may well be because its easier to see mistakes, but if you look at the better classic gear from 50 years ago, it will be seen that construction quality and appearance on the inside was a big deal, and IMO it still is today. But FWIW doing it right in that regard costs money, because you have to pay someone that cares to do it.