I'm playing on a very old Yamaha YBL bass trombone. Back when I played tenor I had very good tone quality, but now in the notes above the staff I'm getting a very sharp tone. Is there anything I can do to get a more rich tone quality on such an old instrument? I did take it in for a tune-up which helped a lot.
Bass trombones take more air than tenors, so loss of tone is natural / may not be down to the state of the instrument but rather the type. To improve your tone you'll need to improve your breathing.
Do exercises such as:
When playing trombone:
If you've tried other bass trombones and believe it really is the instrument that's causing issues, you could try these: