Consider the following VERY PRELIMINARY impressions.
I had a brief one-hour audition of the DMA-300RS, driving the Q7 with the dCS Vivaldi stack as source. I think whoever is considering this amplifier better try them head to head with the 400s for a more informed opinion - I feel they are not just a less-powerful version of them (though it's been a while since I heard the 400s driving the Q7, fed by the SDR-4000SL), some things may be better or more to one's liking, others perhaps not; I didn't have time to compare them. I played the same music I always do, and my impression was that the system had some of the [minor] euphonic colorations of the 360s (though overall a completely different and better presentation), which can be very appealing with certain types of music; but I think I heard a couple of off piano notes, something that I don't notice with the 400s (but different sources); and equally evident with french horns (also absent from the 400s but again with different sources). The difference in sources is reason enough to listen for oneself. Apologies if this is confusing, but can't do it all in an hour; I would certainly like to go back and listen again, because I heard things I never heard before - probably an indication (but not proof positive) that the dCS Vivaldi is superior to the 4000SL (but at what cost!), though there is a slight possibility those off notes were the Vivaldi's doing...
Overall, though the system was quite impressive with RBCD, I felt it really came alive with HDCD and could do a lot better (no time for hi-rez, but just a brief 2-minute exposure to hi-rez while tuning the system left me a little speechless). I think it was a disservice to evaluate this type of system with RBCD but I had to use the same source material for first-impressions, plus time was limited. Turn this comment on its head, and I think it's a waste to buy the Vivaldi stack just for RBCD. With large symphonic RBCD program, I am quite certain I prefer my 400s, even fed by the much-lower-priced Alpha DAC - they are clearly more controlled, natural and effortless.
Switching to Magico for a bit, I realized that the Q3 (based on past auditions) imaged differently (and more to my liking) than the Q7 as set up, while playing Bruch's Scottish Fantasy/Oistrakh. I take this as just one data point, but important enough that it rekindled my desire to get a Q3. And for the umpteenth time, I was reminded that Magico Qs have ALL the qualities of electrostatics and more.
Back to the 300... if I were to compare it to the 260 based on past memories, I'd say go listen and keep the checkbook with you. As much as I love the incredible 260, I felt the 300 is superior in all sorts of ways.
Beautiful stuff, so I will make a point to go back and listen to hi-rez and analog at some point...