Dick,

It would be interesting to compare notes on your trombone setup, since in reality, nothing changed when you moved the tub to and fro along the same pipe. Or did pipe length change??

We achieved very good results with a restrictor pipe (closed off header) dumping into the main large volume canister (tub). Only 100 rpm loss as compared to a tuned pipe.
This system prevents the scavenging gas from entering the header in which a high pressure is present at that time. It is very silent as well.