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My question is this; is the 631 basically a 321 with a compensating valve set? The bells are the same size and, from looking at pictures, at least the first couple of bows look very similar. Yamaha does list the bores as being different- .728 for the 321 and .689/.768 for the 631. They also use different cases, but that might be only to accommodate the 631's side-mounted fourth valve.

I've played several 321's and thought they played pretty well but were ergonomic nightmares. It's tough to tell from the pictures whether the 631 is the same height (Yamaha specs don't list it) and whether the valves are at the same Harley chopperesque ape-hanger level. If anyone out there has played a 631, especially if you've also had some time on a 321, I'd be interested to hear what you think.

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Sorry if I wasn't clearer. I'm trying to compare the BBflat horns, the YBB-321 and the YBB-631. Both of these have 17 1/2" bells. As far as I can tell, the YBB-631 is a rather rare bird on the U.S. side of the Atlantic.

I would agree that the YBB-631 is a great horn, however it depends on what is comfortable to play. There a couple major difference between the two horns. First the 631 has a compensating system, but the 321 does not. Second, the valve material is different(631 monel valves, 321 nickel plated), and they (Yamaha) state the 631 has a dual bore which has an effect on back pressure. I have played on the 631 and enjoyed the experience, but I am more of a C/F player. I would liken the feel of playing the 631 to playing a Besson compensating horn. This comparison only holds to the physical act of holding the horn, and in no way is a comparison of the feel of playing. The horns are similar in size, larger than the YFB-621, but not on the huge size of 4/4 horns.
yes, I believe it is a rare horn to find in the US. If you are looking for one PM me and I will share where I found my rare yamaha (YFB-821).