There's an interesting article over at Positive Feedback Online by Jeremy R. Kipnis of Kipnis Studios, who worked with Bob Katz, David and Norman Chesky and Steve Guttenberg on a number of albums on how they used headphones when making recordings.
(Disclaimer: I write articles for PFO.)
It very much depends on the music, as I have recordings where the level varies considerably. Somewhere around the default "green" volume level though usually. All my RCA cables work with the Hugo's RCA sockets. It is the S/PDIF input that still has a troublesome socket. For that I use the BNC gender changer with my Audiophilleo with a Van Den Hul BNC female to RCA adaptor. Here's my trick set-up with Monster RCA double-adaptors, which are a bit worryingly tight to push on...
I think there is some confusion here. The Studio Six only has one gain level. What you are referring to is the volume setting. The only problem I had was if the volume on the Hugo was too high, where that would result in distortion in the Studio Six, as it only expects a maximum of 2V input (the standard) and the Hugo can output about 3V. That makes me wonder if the volume control in the Studio Six is not on the input as usual, but after a gain stage. I'll have to ask Ken.
The Ref 20s don't fit into the Hugo at all, they have quite large plugs. There are only two versions of the Hugo's case by the way, the latter with the larger holes. I turn the volume on the Studio Six to maximum and use the Hugo's volume control, as at maximum output it sends out 3V, which is too much for the Studio Six.