I’ve had a Vito bass clarinet sitting in my cupboard for a number of years – I bought it, and found it barely playable.
I took it to be serviced by another tech, who seemed to do a good job on the pads – the action felt good, and there was no visible leaking with a leak light.
Yet, the sound was weak and pathetic throughout the range of the instrument, and I could not, for the life of me, work out why. It went away in the cupboard, where it was forgotten about for the last 12 months, until a friend phoned me the other day and asked, “Can I borrow your bass?”.
This prompted me to take it out and have another look at it, and this time I found the problem — Not a leaky key, but a tiny hole in the body, just adjacent the register hole!