Nope, I do get the C in the bass, however, the arrangement for the Bbadd9 is different then for a plain Bb chord. So I end up getting notes mixed in there that I don't want. It's the same as if I want a Bb/G chord, the arranger interprets this as Gm7 and changes the notes of the arrangement to include the 7th degree, but I only want an arrangement of Bb notes with the bass playing G.