They can differ in build quality, appearance, displays, price, and features including sometimes:
- rumble filter
- mono/stereo switch (rare. Never seen one in person.]
- MM vs MC
- variable output levels
- power switch vs no power switch (the only way to power down is to unplug it)
- different cartridge loading or EQ settings (which can alter the tone a bit)
But other than sometimes fairly innocuous differences that don't matter looking at the big picture, I agree: competent ones (even just built-in ones in name brand receivers) usually all sound about the same despite the hi-end reviewers' over active imaginations (and susceptibility to marketing) claiming otherwise.
Occasionally people with cartridges with unusually high output levels can overwhelm/distort some phono preamps not expecting such a hot signal, but I don't get the impression that happens much in modern times.
A common journey phono hobbyists explore is when they discover that with the stylus cued up and not touching the record that different phono pre-amps exhibit subtle differences in noise, hiss. But then when one lowers the stylus into the groove to hear the actual music the noise level skyrockets—from the cartridge hearing the noise of the recording, the disc, the turntable, and the room—and the smart people realize that those faint differences they heard before actual play don't matter because they are almost always obscured, masked, in actual music playback use.
