I just wanted to add my opinion to what has been said about the FW F3 and the HE6. I recently purchased an F3 from Mark at Reno HiFi and read with disappointment reviews on this thread.
I was thinking about returning the amp. Mark was incredibly gracious when I called him, and was willing to take the amp back for a full refund. However, I now have my rig set up, with an EMM DAC2X feeding an AR Ref 5 preamp into a Placette passive preamp and finally the F3. All these preamps in the chain were necessary to get the proper 10K ohm input impedence for the F3.
I am using a q cable with spades attached to the speaker posts of the amp. In any case, I am very pleased with the results. I intend to keep the F3. The set up is in many ways as satisfying as my reference which is Stax 009 with EC Electra.