Went by the Tuba Exchange a couple of days ago and played their two Adams euphoniums, the gold brass and the sterling silver belled horns. Did not do anything quantitative, just qualitative, playing them to see what I thought and felt.
I liked both, but I really liked the sterling silver bell. It produced a big, deeply felt resonance, with a broad euphonium sound unlike any other I've played. The sound is in the euphonium camp, nothing radical, it's just a different mix of fundamental + overtones that I am used to. For me, the sound was neutral on the bright/dark scale. The instrument is light, yes, but the silver bell offsets the lightness, and I do not think I have ever played an instrument which felt as resonant.
I am sure it has its weaknesses, but after playing the horn for a bit, I had to set it down, I was becoming fearful I might make an unplanned purchase....
John