Audiofankj makes a good point. It's always best to listen to amps in your own room with your own speakers. That said, I have heard Nuforce Reference 8Bs against the Rowland 201s and felt the NuForce was definitely superior in midrange and high frequency extension and harmonics. The Rowland 201s were more powerful in the bass and had more power and more slam, but I didn't think they sounded quite right in the midrange and highs.
The latest version of the NuForce Reference 9 seems to have all the bugs worked out of it. It has excellent bass slam and control and the clarity and holographic imaging are strictly top-drawer. I believe NuForce has just extended the warranty on their amps to 3 years and has changed something that makes a "no-load" situation not harmful to the amp.
I've just installed the Benchmark DAC-1 in my system, feeding into the NuForce Reference 9s and the VMPS RM30 speakers and the performance is the best I've heard so far. I'm still in a state of amazement at how uncolored, detailed, and musical my system has become with these latest changes.
But I haven't heard the H20 amps or the eAR 501s, so I can't comment on those comparisons.