LFE setting on Ref 100

[Stereo Steve]

I have just got a MJ ref 100 and initially I'm tring to get away with just a low level input. Fine for movies, got that sorted, Xover at 70 etc.

Now, for SACD, I have set my Denon 2900 for small speakers and a sub which seems to work but if I turn the subs xover to LFE, it goes all blurry and boomy. If I turn it back to 75-80 it sounds fine. Not a problem really but surely my 2900 should not be sending anything over 80khz (anyone where the xover is on the 2900 when set to small?) to the sub. So, a higher setting should make no difference.

I'm not really having a problem with this but I would like to know why this is happening so I can understand sub science a bit better.