This doesn't answer your question (does any question ever get directly answered on audiogon?) but I also have an Nuforce Icon and am now using it as a DAC and preamp driving a tube amp.
I wanted to try tubes in my office system with a pair of JohnBlue JB3 speakers so I bought one of the new Jolida FX10 EL84 integrated amps and am driving it from the line-out jack of the Icon, essentially using the Jolida as a power amp rather than an integrated.
It's a very nice combination, much more organic and realistic than the Icon by itself, and with better bass, but preserving the ability to connect my MacBook to the Icon's DAC through USB. I like it a lot.
It's a much bigger improvement than was adding a Valab DAC to the system to bypass the DAC in the Icon.