The reason the 10 hole is so difficult is because the volume of the hole is
too big a resonating chamber. I wonder if a small piece of blu tack inside
the hole would help. It could be placed at the back to keep out of the air
stream. Be sure to draw very gently! If it worked you could make it
permanent by gluing. I can't try this myself because I sold my xb40 C on
ebay because the top two holes were so difficult. My other xb40s low D, F, G
and A have been used a lot and I'm very pleased except I've had to re-stick
three valves (so far) that ended up in my mouth while playing.
Lurcherjohn