The HP-1 is fine because the voltage rails are high enough and we can always use a lower gain setting. Nwavguy screwed up because the O2 was supposed to be a portable amp, under the expectation that it would be used with portable devices like the Apple iPhone, Sansa Clip, etc. with lower voltage output. The O2 power rails also swing much lower than the HP-1's. Because the O2 ended up more as a transportable / desktop sized amp, folks started to use it with desktop DACs, which caused a lot of these clipping issues. nwavguy was also warned by many others not to make assumptions that everyone would want to use lower voltage output DACs with the O2.