TubeNet

This is probably a long shot, but will a G&W Alan Bear signature Euro shank fit in a 10 year old Yamaha 641?

From general knowledge about tubas and mouthpieces: Yes!

Yamaha is not known to make small receivers, so a European shank should fit, but will of course not enter as deep as an American shank.

Klaus

Yes, but it's not a good fit at all. The school here has those Yamaha 641s (rotary BBb, correct?), so I've given them a try. I wouldn't recommend it. In fact, my Helleberg wasn't an amazing fit either. It's a small shank receiver...

Joseph

I spoke with Mr. Giddings and he said that the euro shank would fit in it if it was a "recent" horn. apparently, Yamaha changed the bore size of this model recently. the thing is, i haven't come across anyone that knows how recent "recent" actually is. i don't know how to find this info out.

I used to own a YBB-641. A Euro shank mouthpiece should be fine. I was able to use Miraphone mouthpieces (slightly larger than standard/American shank) in it with no problem.

These horns pretty much sound the same no matter what you stick in them.