I have an old 1962 conn 18h with a copper bell that has a .484 bore. I am generally playing lower pieces that don’t sound as good as the high register. Is there any way (other than a new mouthpiece or instrument) to get a fuller tone?
You can get a fuller and rounder tone with more volume if you make your lips round, thinking an big Oh ... in the beginning you will lose much air but with more practise you will improve your sound.
There is a reason why Conn offer instruments like the 18H (.484) and the 88H (.547). They sound different!
Whether YOU are getting the best possible mid/low range sound out of the 18H is another matter. We can't really advise on that without seeing and hearing you play.