I happened across these:

http://www.erseaudio.com/Products/SPXProSpeakerCabinets

and wondered if they could work as Pi 4 cabinets. I'm not
sure what your current volume is, and it looks like the built-
in flare is narrow compared to yours, so maybe the crossover
might be a touch higher. The price seems fair if they could
give a good result.

take care,

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