Logic Pro 9 5.1 Surround- panning issues. Volume dip in phantom rear center

Please help
When I pan around in a 5.1 stereo field there is a huge dip in volume in the middle between the rear L and R, where the phantom rear center image should be. Imagine a -10db pan law (just in the rear) to get the idea of what I mean, or a missing rear center speaker in a 6.1 system.
I am using setting 5.1 ITU 775 (default and WG-4)
Connected up using an MOTU 828, using separate discrete outs from the soundcard to discrete channel inputs to my receiver (amp) harman kardon
I have adjusted my pan law to 0db to try to correct, but no change.
Has anybody else noticed inconstant panning issues (in the rear) like this or have any ideas what I may be doing wrong?
This issue seems to make logic pro a bad choice for surround mixing.
Two questions:

1. Are your monitors identical, matched and approved for 5:1? What are you using?

2. Has your control room been acoustically prepared for 5:1?

This issue seems to make logic pro a bad choice for surround mixing.

FWIW, I have carried out fairly extensive comparison tests between Logic in 5:1 and Nuendo in 5:1, in both cases I detected no differences in Monitor response. Have you measured the response in your surround field, or is the huge dip you are referring to more how you hear the response?

kind regards

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Thanks for your reply. All my speakers are the same. Its part of a 5.1 surround sound package by harmon kardon.
The space is semi acoustically treated. However I did do a test, by taking it out og 5.1 surround and set a channel to output 3 and 4, which corresponds to rear speakers Left and Right. Then I panned just the rear speakers with no problem. Thus eliminating the room acoustics, and speaker problem out of the equation.
I am still stumped to why the rear surround behaves so differently...
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