I've not personally been involved in helping an instrument-maker develop his/her ideas, but I recently played a violin that was the result of much collaborative research:

www.themonteverdiviolins.com

If I had bags of money I'd be getting one of these.  Great sound and response, and it did blend very well with the other one.  So it's on the list for my next lottery win (along with a late classical/early romantic era violin, a good modern bow, an early 17th century bow...) 

There's a terrific bowmaker in Germany who also bases his designs on a combination of careful measurements of originals and paintings from the time:

www.bows-viols.de/index_eng.htm

I know this approach is standard for most makers of early instruments, but some seem better at it than others.  I suppose you have to choose your sources carefully.

And then there's the question of tweaking authenticity for practical purposes.  My violin, for example, is similar in size to the 1605 Amati brothers violin that George Stoppani used as his model.  It's from slightly later (1650-ish), but would probably sound wonderfully appropriate in a similar very early set-up.  But it's my only violin, and I need to play Bach, Handel, Mozart etc on it.  The luthier who restored it discussed this with me, and we decided to use a later model of bridge and a longer fingerboard than it ought, strictly speaking, to have (though I do string it at equal tension).

Yes, yesterday's gig Tommo.  I think it was fine - but my ears stubbornly remain blocked.  I could still tell a good violin through the muffle, though.  I'm going to beg my friend hard for a chance to try the violin when my ears are actually working properly.

Very interesting. I've read some very similar things from Peter Holman in connection with a set of instruments he commissioned (?) for his own ensemble, which do sound very convincing on some of the recordings I've heard.

The twentieth century seems to have been a fallow period in terms of creative interaction between composers and instrument-builders (compare with Wagner and Berlioz, let alone earlier composers), owing no doubt to the quasi-industrial standardisation of instrument technology in that period. More recently, though (just as an aside), the situation was begun to change again, since in the domain of electronic music (I mean the sort that's performed rather than just played back) the roles of composer and instrument-maker often become indistinguishable: "building" an instrument consisting of computer software and a physical means of actuating it is pretty much the same thing as deciding what sort of music you want to be able to play on it.

Maybe I should put it in stronger terms then . what I meant was that composing the music and designing/building the instrument become the same thing, done more often than not by the same person.

Except, perhaps, in the sphere of percussion?  Look at the "kitchen department" as it stood at the end of the C19th..  there is very little indeed, especially in the sphere of pitched percussion...   whereas composers now expect that exotica like a bass marimba will be "on call" almost anywhere.   I know they are possibly atypical, but Partch comes to mind too, and I may even for once beat Bryn into naming Nancarrow in this context?

Oh, and Scriabin's "colour organ".  And the Ondes Martenot. 

I suppose the biggest change in instrument technology in the C20th was affordability...  the technological leap Richard mentioned meant that even modest students can afford a fine playable instrument, compared to the cost/quality of such an instrument in the preceding century?

Indeed. The thing with a lot of this technology is it's made so that as soon as you have it up and running some vaguely interesting stuff can be made to come out, unlike with violins and clarinets (they just aren't user-friendly enough!), which, if "vaguely interesting in the same sort of way as loads of other people who have been here before and can't be bothered to go any further" is one's bag, is already enough. Otherwise you have to start thinking and learning and practising.

Adrian Brown, to be precise.

http://www.adrianbrown.org/recorder_types/

You'll probably have to click on 'Ganassi' recorders in the menu.