Anyone have experience with the Yamaha SL-51BL or Yamaha SL-51C4L as compared to a Bach 5, 5GL or 5GS?

The Yamaha 51B is pretty shallow, comparable to a Bach 5 with a larger throat. For orchestral solo work, perhaps?

I tested a 51C4 against a Bach 5GS when I was looking for a piece for my large tenor; for me the Bach came out on top. I thought the Yamaha was "perfectly playable if presented, but" a little duller in sound quality, not as "interesting."

The 5GL is a little deeper than either of the above, and for me provides the best sound out of my large tenor. Ymmv.

If you want to go Yamaha on a large bore bone, I think the plain Yamaha 51 works better then the smaller alternatives 51B and 51C4. Actually, I think the plain Yamaha 52 is perfect for my YSL 682B large b on re Bousfield. Depends what you want to archieve, though.