  We work out constraints imposed by channel duality and analyticity on
tree-level amplitudes of four identical real scalars, with the assumptions of a
linear spectrum of exchanged particles and Regge asymptotic behaviour. We
reduce the requirement of channel duality to a countably infinite set of
equations in the general case. We show that channel duality uniquely fixes the
soft Regge behaviour of the amplitudes to that found in String theory,
$(-s)^{2t}$. Specialising to the case of tachyonic external particles, we use
channel duality to show that the amplitude can be any one in an
infinite-dimensional parameter space, and present evidence that unitarity
doesn't significantly reduce the dimension of the space of amplitudes.
