If we speculate that the REF 10 replacement may stand in the same relation to the REF 6 as the REF 10 did to the REF 5SE. What we may see is:
A line stage that is basically two REF 6s with separate power supplies in one box where those power supplies give each channel the equivalent of the entire REF 6 power supply. Essentially the REF 10 is dual REF 5SEs, just as the REF 10 Phono is two REF 2SEs - one for each channel. I'd be surprised if ARC took a different route with the REF 10 replacements.
In 2017 I had the opportunity to have the REF 2SE, REF 3, and REF 10 Phono available to my system simultaneously, with the Line Stage being the REF 10. I never wrote that REF 3 phono stage review , but from past experience I can tell you the REF 3 and REF 6 are quite similar to one another in their basic sonic characteristics, just as the REF 2SE and REF 5SE are to each other.
Back to speculation on the REF 10 replacement. If the preceding holds true, then its not hard to extrapolate the sound of the REF 10 replacement being a highly refined version of the REF 6 sound - at the same level of the difference between the 5SE and 10 (same gain stage circuit.) When I did the above comparison it was easy to hear how the REF 10 Phono derived from the 2SE, and as Ken noted in his excellent account of the 10 and the 6, the REF 6 had a "very different sonic signature."
However, what I wasn't expecting was the REF6 preamp's very different sonic signature. The soundstage was much more forward than the REF10, the attacks were more abrupt and the mid-bass was punchier, creating a very vibrant and more aggressive sound. In some ways this provided more immediacy to the music but at the expense of some edginess compared to the REF10's silky smooth midrange and treble. The best single word I can use to describe the REF6's sound is "ballsy", while the best single word to describe the REF10's sound is "refined".
The presentation of the REF10 and REF6 were different enough that I would say it comes down to a matter of personal taste as to which one a listener would prefer. While I really liked and could be happy with the overall sound of the REF6, being a lover of jazz vocals, jazz piano, jazz guitar and small jazz combos, the REF10's sound really did it for me.
So the message here is .... be careful what you wait for. While I do not believe that equipment is music genre specific nor would I say it in a formal review, my gut tells me the REF 10 replacement could be 'more adept' with pop and rock. The REF 3 was bit more dynamic and a bit more detailed than the REF 2SE, but along with being more foward the 3's tonality and harmonics were, imo, not quite as genteel or refined. Compared to the 3, the REF 10 Phono was more laid back - that's not a negative but a difference in approach. Imo, based on direct comparison, ARC changed the sound of the lower member of their upper tier line with the REF 3 and 6. It's a personal choice of course.
Top row: REF 2SE, GPA Monaco 2.0 TT
Row 2: REF 10, REF 3
Row 3: REF 10 PS, REF 10 Phono PS, REF 10 Phono.