Dear Neil, very nice system you own.
As you know I always support the two subwoofers in true stereo fashion and I don't want to go on that way but what I want to ask you about is if you already try a crossover frequency a little higher than 35hz, say it 60-70hz.
IMHO I think that with a little higher frequency you could take way lower the intermodulation distortion on your ML and if this could happen then you " clean " your whole sound reproduction.
Well I don't know the operation facilities of your sub, I'm assuming that has independent low and hig pass filters.
Again congratulations for your very fine system!!!!
Regards and enjoy the music.
Raul.