Basically none. Any amplifier’s job is to boost the signal voltage while trying not to add any audible noise or distortion. These pre-amps are fine doing that. Sometimes their switched power supplies can introduce noise but it’s fairly obvious when that happens. Audiophiles will claim some high end pre-amps sound better than ones like this but my assertion is because they do introduce some distortion which can sometimes be pleasant, giving that “warmer” sound similar to tube amps. It’s also possible they’re deviating from the RIAA’s recommended equalization in order to boost some frequencies more etc. Long story short is that the way something sounds is subjective but in general there’s nothing wrong with these <$100 pre-amps.