Without knowing how the S/N ratio was calculated (ie. at 1 watt or 100W) it's tough to say whether it is high but the 100db level does suggest a high ratio, which has been a trend in receivers and processors lately.

Can't say for sure but that could be an issue with your 350 and may explain the auto-switch problem? It could also be a reason why some people say their EP subs have little sensitivity in the gain control.
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John