Volume knob... line mixer?

[Joost Schuttelaar]

So I got my new desk setup, and dusted off my Tannoy Active Reveals to use again as monitor speakers. One thing: they don't have a level pot, just an on/off switch. I wanna add something so I can easily control the volume from my MacBook Pro (not software based...). What's small and decent, to hide somewhere on the desk? Don't need to plugin anything extra, although I'd probably get a small line mixer too in the near future for my synths (get rid of the big rack one).

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Joost Schuttelaar
The Hague, NL