The LPF for the LFE in the Denon X1400 looks like just a crossover, if I am reading
the manual correctly.
For desktop PC setups, in my experience, subwoofer localization can be more prevalent since the speakers and subwoofer are so much closer than in a room. A way to reduce localization effects is to use a lower crossover point (or multiple, spaced-apart subs). I would say he should experiment with different frequencies and just keep whatever sounds best to him. Bass localization might be an issue for him or it might not be.