Of course being a native bass and tenor clef reader, I read transposed Bb treble the same way I read tenor. I just add two flats. Actually, I now just read treble and think concert pitch instead of transposed. So with written D's I think concert C, etc.
By the way, I studied cello and baritone growing up and learned to read tenor through my cello studies. I also got proficient in tenor when reading some trombone solos and etudes. The fact that tenor is a "native tongue" for me helps me with Bb treble, but I still sometimes mess up accidentals or key changes. I have to be careful. It helps that I try to use treble euph parts in the wind band most of the time so that the transition to British brass band is helped. After 8-9 years of lots of treble clef, I am mostly pretty good now. BUT, darned if the semi- hemi- demi-quavers, crotchets, and minums don't get to me when I go to the UK every summer!
Doug
Adams E3 0.60 Sterling bell - Prototype top sprung valves
Concord Band
Winchendon Winds
Townsend Military Band