The
Library!?! 
That's a new one on me for rehearsal space! What? A sound treated room in the Audio Section, or something? My college Student Union had that kinda thing many, many moons ago. I never saw them used that way, but seems to make some sense now you mention it.
And that no-amps rule makes sense, too. Especially since bass frequencies propagate through buildings so easily! Even something like the low power Fly might be able to offend there. I dunno.
You may have found the place for an unamplified acoustic bass guitar! Although, you
may struggle to hear it yourself, much less the others. And, what are the long term prospects of continuing there that would justify that kind of expense?
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AC1 Rhythmvox Bass - no longer in production, used market, possibility

VX50BA - no battery power, way overpowered

PB10 - no battery power, I think, possibly overpowered?
- See above reference to bass frequencies propagation.
Just my 2¢. Good luck.
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