  Due to the absence of hadronization effects and the large top mass, top quark
decay will be uniquely sensitive to fundamental electroweak physics at the
Tevatron, at the LHC, and at a future linear collider. A complete measurement
of the four helicity amplitudes in top decay is possible by the combined use of
Lambda_b and W polarimetry in stage-two spin-correlation functions (S2SC). In
this paper, the most general Lorentz-invariant decay density matrix is obtained
for the decay sequence (t --> W b) where (b --> l nu c) and (W --> l nu), and
likewise for the CP-conjugate mode. These density matrices are expressed in
terms of b-polarimetry helicity parameters which enable a unique determination
of the relative phases among the helicity amplitudes. Thereby, S2SC
distributions and single-sided b-W-interference distributions are expressed in
terms of these parameters. The four b-polarimetry helicity parameters involving
the A(-1,-1/2) amplitude are considered in detail. Lambda_b polarimetry
signatures will not be suppressed in top quark analyses when the final
neutrino's angles-and-energy variables are used for (b --> l nu c).
