King or Conn compensating euphonium
I playtested the Kanstul and wasn't impressed with it, though I do own and play on a Kanstul 33T, their non-comp BBb valves-across-the-top tuba. I wasn't impressed with the ergonomics on the Kanstul. Short, stubby, it just didn't fit well. Sound quality wasn't good, either. Sort of tinny and I had the impression that there just wasn't enough metal in the bell.
As to the rest of the American companies, there might be a point at which the market wouldn't support another company making comp euphs -- I dunno.
Good topic for discussion, though.
U.S. Army, Retired (built mid-1950s)
Adams E2 Euph (built 2017)
Boosey & Co. Imperial Euph (built 1941)
Edwards B454 Bass Trombone (built 2012)
Boosey & Hawkes Imperial Eb tuba (built 1958)
Kanstul 33-T lBBb tuba (built 2010)