Whether someone can hear aliasing or not is complicated. It's a psychoacoustic thing.
It depends on the exact nature of the aliasing. When the Kemper first came out the aliasing was very noticeable because the aliased tones occurred in the midrange when playing higher notes. If the aliases are near the distortion products they are masked by a phenomenon known as perceptual masking. This is the same phenomenon that is the basis for MP3 encoding. Humans can't hear quiet tones that are close in frequency to loud tones.
As with all engineering problems it's a trade-off. To reduce aliasing you can either increase the sample rate (via oversampling, for example) or use Antiderivative Anti-Aliasing (ADAA). Both of these approaches increase the amount of CPU required. Simply oversampling is very effective but doubles the amount of CPU cycles.
ADAA can achieve aliasing reduction but it's no free lunch either. The problem is the antiderivative is typically a more complex function, sometimes much more complex (tanh(x) becomes log(cosh(x))or example). For example if the function is, say, x^2, the anti-derivative is 1/3*x^3. This requires two extra multiplies. You might as well simply double the sample rate. Then you have to take do a difference and if the difference is near zero you risk generating a result that exceeds the numerical range so you have to add conditionals which slow things down even more. ADAA also doesn't work for nonlinear systems with memory. Nearly all our algorithms employ memory so we don't use ADAA.
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