Has anybody else managed to achieve low-pass or high-pass effects using the empeg DSP?

Do NOT use the equalizer as a high pass filter or a low pass filter. It is not a useful substitute for a proper crossover circuit. It does not work well at all when you try to do this. It only attenuates to a degree, it does not do a complete shelving cutoff.

If you need a proper crossover, use a proper crossover. Most amplifiers have proper crossovers built-in.

I'll add this information to the FAQ entry.