  The quasi-one-dimensional (q1D) material Rb$_2$Mo$_6$Se$_6$ has been proposed
to display a nontrivial combination of low-dimensional fluctuations and a
dynamical charge density wave (CDW) at ambient pressure. This may lead to a
progressive metal to insulator cross over at low temperature. To explore the
link between the crystal dimensionality and this insulating instability, we
have performed hydrostatic pressure-dependent electrical transport measurements
on single crystals of Rb$_2$Mo$_6$Se$_6$. At low pressure, we observe
thermally-activated behaviour consistent with a temperature-dependent gap
$E_g(T)$ opening below a characteristic temperature $T_{Rmin}$. Upon increasing
the pressure $T_{Rmin}$ initially rises, indicating a reinforcement of the low
temperature insulating state despite a continuous reduction in $E_g(P)$. We
interpret this as a signature of suppressed fluctuations as the dimensionality
of the electronic structure rises. However, $T_{Rmin}$ drops above 8.8 GPa and
superconductivity emerges at 12 GPa. Between 12-24.2 GPa the superconducting
and insulating instabilities coexist, with superconductivity surviving up to
the maximum attained pressure (52.8 GPa). Analysis of the magneto-transport
reveals two distinct regions: at high pressures the anisotropy gradually falls
and the superconducting state appears unremarkable. In contrast, coexistence
with the gapped insulating phase creates a superconducting dome. The emergence
of a peak in the critical temperature Tc despite the depleted density of states
is indicative of enhanced coupling. Our journey from the extreme 1D to 3D
limits in this prototypical q1D metal reveals an intriguing relationship
between superconducting and insulating ground states which is simultaneously
competitive and symbiotic.
