  Here, we investigate the nonreciprocal propagation and amplification of
surface plasmons in drift-current biased graphene, using both Galilean and
relativistic-type Doppler shift transformations of the graphene's conductivity.
Consistent with previous studies, both conductivity models predict strongly
nonreciprocal propagation of surface plasmons due to the drag effect caused by
the drifting electrons. In particular, the Galilean Doppler shift model leads
to stronger spectral asymmetries in the plasmon dispersion with regimes of
unidirectional propagation. Remarkably, it is shown that both conductivity
models predict regimes of nonreciprocal plasmon amplification in a wide angular
sector of in-plane directions when the drift-current biased graphene sheet is
coupled to a plasmonic substrate (namely, SiC), with the plasmon amplification
rate being substantially higher for the relativistic Doppler shift model.
