Greetings,
I can't comment on the PH6 but I have experience with the PH5 and PH7. I still use the PH7 and really like it. I would choose it over the PH8 because it works better for my needs, i.e., it allows me to tube roll with all its tube types. That helps me get what I'm after rather than what ARC is after, which sometimes isn't my cup of tea. Stock tubes, whether 6922's or 6H30's are okay but can be bettered!! Many of the people who sell ARC gear in my area have never heard it with non stock tubes. A real pity. The ability to choose between 6922's is a must for me and something I wouldn't give up by going to something like the PH8, or any unit that forced you to accept so few options. If your system is perfect sounding than a PH6 may be a good fit.
I have heard some of their newer gear in a local showroom and have liked it, though it was very limited listening time and was with better speakers (way better) and a better sounding room than mine.