Well I said I'd report back once I'd got my Harley Benton fretless violin bass, and it was delivered yesterday.
It looks gorgeous, but was largely unplayable straight of the box, due to a convex-bowed fretboard causing serious buzzing on all lower notes. However, a quick gen up online and I was ready to do some gradual truss rod adjustment - it took four loosening 1/4 turns to do the trick in the end, getting rid of all the fret buzz by converting it to a slightly concave arc (this stuff is all new to me
I've now got some £5 feeler gauges arriving via Amazon this afternoon, to further tweak the action, but thus far I'm well pleased (apparently you need a capo on the 1st fret, your fingers pressing down on the fingerboard where it joins the body, and then a fretboard/string gap of around 0.015 inches/.381mm at the 12th fret position).
I suspect I may need help to deepen my nut slots (these require a set of special files the same width as each string, and needs doing very carefully!), but I'm not exploring that option until the truss rod is perfected, as I don't want any return of the buzzes.
I'll provide more feedback on the tone and playability once the action is fully sorted.
Oh, and Wonks, you're right about me probably needing a 1st fret marker (it's at 43.5mm, thanks to your fret position calculator), as it's a huge gap between the nut and the first 3rd fret marker! I'll see how I get on by ear first.