Graham, the symptoms that you describe (the B1 being initially easier but becoming comparatively harder as you tire) indicates that you might benefit from an incremental mpc change. Talk to Brian Scriver for the best advice.
You sensed my affinity for the B1. It's my favorite Schilke and I was seriously considering a B1L when I found the Concept TT. The TT has a more open blow than the B1 and feels more dynamic. I found that the TT has the resonance of the B1L with the security of a fixed bell. Both horns have astoundingly smooth, fast valves and first rate construction. Oh yeah, with Selmer you're not forced to buy silver.
Dave