+1 to the HPF concept - on damn near everything except bass & drums. But I nearly always use a LPF on amp sims, too.
Amp sims introduce harmonic distortion, just like a real amplifier. That means they're adding high-frequency content. But real amplifiers have a built-in lowpass filter in the form of power amplifiers and speakers that simply cannot reproduce high frequencies. The result is a rich midrange without that annoying "fizz". In a digital emulation, no such restrictions exist, so those harmonics go right on up to fs/2 unless you add a LPF.