I am a speaker designer and could make almost any size woofer "work", but when I did it I chose a 12". (it works a treat, you wish you had it!)
Can't speak to other's designs in general, as there are too many parameters to make a flat statement.
Guitar bass strings go down to about 80Hz, and you want a smooth low-end roll-off with no lumps and good control by the amplifier device.
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-donh-
*everything* is a tone control
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