  The multichannel Efimov physics is investigated in ultracold heteronuclear
admixtures of K and Rb atoms. We observe a shift in the scattering length where
the first atom-dimer resonance appears in the $^{41}$K-$^{87}$Rb system
relative to the position of the previously observed atom-dimer resonance in the
$^{40}$K-$^{87}$Rb system. This shift is well explained by our calculations
with a three-body model including the van der Waals interactions, and, more
importantly, the multichannel spinor physics. With only minor difference in the
atomic masses of the admixtures, the shift in the atom-dimer resonance
positions can be cleanly ascribed to the isolated and overlapping Feshbach
resonances in the $^{40}$K-$^{87}$Rb and $^{41}$K-$^{87}$Rb systems,
respectively. Our study demonstrates the role of the multichannel Feshbach
physics in determining Efimov resonances in heteronuclear three-body systems.
