I've just the other day compared the Echo AudioFire 4 with the ProFire 610 (both have THAT1510), and I found that the ProFire sounded smoother, fuller, a bit more 'spacious', and resolved reverb tails tails more completely.
The M-Audio Fast Track Ultra (not the Pro) also has 1510s; it was third in my ranking (a bit 'coarse' by comparison) .