Sim Audio I-5 vs. new I-3---sonic differences?

I believe you are right in your thinking. I spoke with a dealer regarding this. His opinion was that the the i-5 was
superior sonically. Another possibility you might want to consider are the Manley EL 84 based mono blocs (50w per ch)
One of these two products will be my new amp when I change my present Bryston B-60R. My feeling is that the extra money for the i-5 would be well spent.

I owned a Moon I-5 for over a year and found it to be very well made, easy to use and integrated well with my subwoofer. Simaudio is also a great company to work with. However, I also was never able to get it to sound right with anything I tried it with. I'm not saying it always sounded bad, quite the contrary, sometimes it was magical. But in my room it had a tendency to be lean and a bit sizzly on top no matter what cables or speakers I used it with, and over time that became unbearable. I eventually went to a tube preamp mated to a solid state amp. I think that move in itself may tell you that my tastes could be different from yours, but I thought I would just offer my experience.