These are great little amplifiers within their limitations.
The problem is, they're not really hi-fi. Solid-state amplifiers are expected to have distortion way down <0.001% nowadays, particularly for headphone use, where the power is often no more than a milliwatt. At these power levels you're better off with a high-current opamp, which can have ultra-low distortion. This is not to say that you'll necessarily notice the distortion, it's just that you can get so much better chip amps if that's what you want.
So they're often found in intercoms, cheaper powered computer speakers and suchlike. You can get 10 watts using 2, bridged, which is quite a respectable output for a guitar practice amp, but the sound is a bit flat and lifeless without a lot of tone generating and shaping circuitry in front, and of course you need ~25V, which is not that easily come by, although a wallwart may come close.
I have a few lying around, but I don't expect them to get used for anything, I just don't like to chuck them out. You never know...
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