If all your problems started after that power supply replacement, this further reinforces the idea of stable power. You seem reluctant to believe that a new power supply can be a problem (it can). For what it's worth, 13 drives x 5w equals 65w, that shouldn't be a factor.
In any debugging and recovery operation, the objective should be to manage the change rate and therefore risk. Replacing the whole system would violate that strategy.
Do the drive connectivity failures implicate a SAS card problem? Maybe, but a much more plausible explanation is physical connectivity or power. If you have an identical SAS card, and it is passive (no intrinsic configuration required), replacing it is a low risk troubleshooting strategy.
Do failures implicate the motherboard? Maybe, if you are using on-board SATA ports, but the same plausibility test applies. However, there is more variability and risk (mobo model, BIOS settings, etc).
Do failures implicate DSM or loader stability? Not at all; DSM boots fine and is not crashing. And if you reinstall DSM, it's very likely your arrays will be destructively reconfigured. Please don't do this.
So I'll stand by (and extend) my previous statement - if this were my system, I would change your power and cables first. If that doesn't solve things, maybe the SAS card, and lastly the motherboard.