Bright switches typically add upper mid/treble content (usually a fixed boost either in the upper mid area around 2K, or in some amps, in the treble area around 4K). Some bright switches, like on the EA amps, are set WAY up in the upper treble region.
They are typically useless, and were designed mostly to either 'make up for dead strings', or to add at least a little bit of sparkle to darkly voiced basses.
If you have a bass with really old strings, or a very darkly voiced bass, combined with a cab without a tweeter, it is quite possible that you would not hear the difference between this type of switch being engaged or not.
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