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The bryston will be fine with a lower load. I don't know what the back is talking about. I have a 3B and never had any problems. I was looking at some Totem Mani-2 once and Bryston said it would be fine even at a 2ohm load.
As far as the date email James Tanner at bryston and he will be able to help you out. They have a 20 year warrenty from the date of manufacuring. I am not sure how old your amp is. If its getting old I would send it back for a free servicing.