Mark Levinson 380S or 320S ?

For the same price, without doubt the 320s.
I newer and for me better design and perfomance.

I'm not quite sure what unit you listened to but if you think the 320S was lacking on the low end, you need to reexamine your system because you have problems elsewhere.

I am going against the flow BUT do consider the current Classe pre-amps, CP-500 and CP-700. Having owned both the 380S and 320S, my conclusions are the 380s is very dark (dulls the musicality) while the 320S (IMO) lacked solidity and heft in the bass notes (maybe on purpose?). In terms of value, the CP-500 easily bests the 380S while the CP-700 is the pinnacle in pre-amps from mainstream HI-END manufacturers (exotics are excluded). My reference is actually a Theta CB3 with Extreme D-2 DACs (for digital) and pure analog pass through for non-digital sources.

2nd the 320S.... newer design with nicer boards & improved sonics for sure.

320S hands down.