  In this paper, we study a range of collision systems involving deformed ions
and compare the elliptic and triangular flow harmonics produced in a
hydrodynamics scenario versus a color glass condensate (CGC) scenario. For the
hydrodynamics scenario, we generate initial conditions using TRENTO and work
within a linear response approximation to obtain the final flow harmonics. For
the CGC scenario, we use the explicit calculation of two-gluon correlations
taken in the high-$p_T$ ``(semi)dilute-(semi)dilute'' regime to express the
flow harmonics in terms of the density profile of the collision. We consider
ultracentral collisions of deformed ions as a testbed for these comparisons
because the difference between tip-on-tip and side-on-side collisions modifies
the multiplicity dependence in both scenarios, even at zero impact parameter.
We find significant qualitative differences in the multiplicity dependence
obtained in the initial conditions+hydrodynamics scenario and the CGC scenario,
allowing these collisions of deformed ions to be used as a powerful
discriminator between models. We also find that sub-nucleonic fluctuations have
a systematic effect on the elliptic and triangular flow harmonics which are
most discriminating in $0-1\%$ ultracentral symmetric collisions of small
deformed ions and in $0-10\%$ $\mathrm{d} {}^{197}\mathrm{Au}$ collisions. The
collision systems we consider are ${}^{238}\mathrm{U} {}^{238}\mathrm{U}$,
$\mathrm{d} {}^{197}\mathrm{Au}$, ${}^{9}\mathrm{Be} {}^{197}\mathrm{Au}$,
${}^{9}\mathrm{Be} {}^{9}\mathrm{Be}$, ${}^{3}\mathrm{He} {}^{3}\mathrm{He}$,
and ${}^{3}\mathrm{He} {}^{197}\mathrm{Au}$.
