It is best to have it connected at both input and output.
Got the Ref-8 today. Initial impressions as compared to my Ref-1 are that they are very close (Ref-1 via ACSS to Phoenix, Ref-8 via XLR to Phoenix- as preamp to my speaker rig consisting of a Emotiva XPA-2 amp and Mirage M7si speakers). My test for treble is with Cara Dillon as there is some sharp sibilance in her recordings. The Ref-1 is always smooth but compared to the Ref-8, the sharpness of her voice is very slightly more present (really small difference). Both sound spooky real with her though. There is definitely no obvious tell-tale of "warmness" vs. "clinical" between the two IMO.
I feel I need a couple of hundred hours of use before commenting further.
From what I've experienced; Emotiva amps, especially the XPA series, are on the cold and clinical side of neutral. On the other hand, they are also very transparent. They will reveal component changes but add it's own degree of sonic signature to the mix (cold/dry, a bit edgy/strident, and some over emphasized bass). I think it's a good amp for component evaluation as long as you know what it's adding to the overall sound. Not sure what you're using as a transport, but a Virtual Dynamics digital cable will greatly help with making sibilance more natural and less like added noise. It will also make the presentation more fluid. Happy listening.