With regards to the Fulla,it is a little weird to use what is probably an inferior amp section instead of the amp on your mojo just for a volume control. You might be better off with a nicer amp without a DAC and just using the mojo as a DAC. If you don't like the volume buttons on the mojo why not just sell it and use the Fulla or a JDS element as both a DAC and amp. If you are really in love with your Mojo amp section, then the SYS would theoretically let you use its amp section since it just provides passive attenuation. The SYS doesn't have a headphone out since it's a pre-amp, but I guess you could get a rca male to 3.5mm /6.5mm female connectoion if you wanted a knob instead of buttons. If all you want is a knob though you might be better off with something like the emotive control freak if ergonomics are reall important to you. The SYS knob is functional, but honestly not that great in terms of ergonomics; it's a small knob on a small unit. You could also look at something like the PAC-LC1 which is just a (linear) volume pot inline in RCA cables if you want a really cheap option. It does use a linear taper, so that's the main downside to that option.