It may indeed be the powering but that PSU should be very confortable running 2 S3's at stock (even overclocked within reason!).
That you get an x when you clock to 225 is rather telling though. If this is one of the very early S3's, did you get a chance to re-apply the heat paste?
I mention this because with one of my upgraded S3's (from S1), I used heat-pads rather than paste on the chips, and intitally some of the pads moved slightly from the chips when I was adding the heatsink; that unit kept "throwing up" and x now and again, and after updating to 4.6.1 it became a "main-stay". I decided to check the rig, unscrewed the heatsink and discovered what I mentioned above, aka the slipped heat-pads. I put them in place and added a bit of paste to the back and the x is now history (well, its only been a few days, but it has not appeared again ..... yet).
So it may be the case that if you have an early S3 (which were notorious for too much paste or non at all!) AND the unit was hashing OK before applying 4.6.1 then it is worth investigating that, otherwise I'd say 4.6.1 does not suit that unit.
Yes, it is one of the early units and when received, thermal paste was splashed all over the asic connectors. After cleaning, I first tried Artic Silver, which I use when o/c CPUs. This gave me the lowest temps I have seen with this S3, but after scare stories on the S3 o/c thread about Arctic Silver being electrically conductive, I removed it and applied Prolimatech, which increased temps by 1 degC compared to Arctic Silver. I wasn't impressed by that, so bought some Zalman paste to try, which comes in a little bottle with a brush - easiest to apply by a mile, but quite expensive. This gave the same temps as the Prolimatech. TBH, I wish I hadn't removed the Arctic Silver now, although it's horrible to apply as it tends to trail as you lift off the applicator, so massive care is needed.
Anyway, to try to get rid of the 'x', I redid all of the paste this morning, using the Zalman, as it's the quickest. Unfortunately, the first time I ran at 225 again, the 'x' showed up in chain 1. Had to reboot to get rid of it and reset to 218. Just out of interest, I then tried it at 231.25. After about 30 minutes, I got an 'x' again, but in chain 2 this time! The chain 1 'x' has not re-appeared after 1.5 hours at 231.25. Is there any risk, do you think, running with an 'x', because it seems to run very well at 231.25 apart from that? Oh, BTW, I'm also running the S3 on its own Enermax PSU now, although as you suspected, I don't think that's the problem.