Could you? Assuming it’s a Gen 1 PLAY:5 that actually has a headphone out, yes, probably, but I wouldn’t recommend it, as it’s often not useful to supply a variable level output to an amp of any kind. Difficult to set the appropriate level to match what the Amp is looking for.
Later versions of the PLAY:5 dropped the headphone output, and only have a line in.
Notwithstanding the level/impedance mismatch between the headphone outlet on the Play:5/g1 and an amp’s line-level input, in my experience it has relatively indifferent signal-to-noise performance, which would only be magnified by the house’s ‘really good sound system’.