S'funny, I thought we ended up debating whether 37 or 31 was the way to go.
I have a short recording of the twins and the most obvious thing is that they 'chug' and the turbo is pretty much inaudible. So I'm convinced a 40 or 50 (or 56!) is not the way to go.
I think the 50 recoding is used by either SWD or Howes for their '10000' profile.
When I've got a few free moments I'll have a look and see if I can find the previous thread.
Steph