Yes, Feri and I disagree on this. But I am the one who ran his AVR at +10dB and destroyed a sub because of it. Following Feri's line of thinking +10dB ought to be fine, but I know from experience it isn't. I can only tell you what I believe to be true - the sub trim should ideally be well clear of the maximum 'end stops'.
Keith, did that poor little sub at +10 dB trim level deliver an SPL of 85 dBs at the MLP?