It produces very audible harmonic distortion for anything that is above -6 dB. Better disable it.

How to verify: generate a 1 KHz tone at -1 dB or -2 dB with any audio editor, and save it. Play it with Foobar2000 with and without the hard limiter activated. Very obvious difference.

if you prefer clipping with real music to distortion with some artificial signals, go ahead and disable it.

Distortion will happen with any kind of signal that goes above -6 dB, in practice any modern CD track. The 1 KHz signal is just an easy way to check it. I think that it would be easily ABXable with regular music, it produces a kind of coloring soft distortion. Replaygaining is a better solution for the problem of clipping. Even if no replaygain is used, I prefer occassional clipping on occassional songs, which for me is not annoying unless it's very strong, that a permanent distortion of loud signals. Note also that clipping may happen only with lossy compressed files, and not with losslessy or not compressed files, but the hard limiting distortion will happen with any kind of sound files.

On the other side, everything else seems to be very good in your player.

Edit: another good'n'easy solution for avoiding clipping would be simply to set the preamp to -1 dB. This way all possible clipping is almost surely avoided, and the 32bit fp engine will take care of the rest.

set volume to -6dB and use external amplifier. better ?

I just edited my post before you replied. I agree, that is a better solution, but I think that in practice -1 dB or -2 dB as much, is enough.