I assume you are using the 5.1 anologue inputs, if so you could try a 'cheat', as described below. I tried it, found it worked, but was not to my liking.
Get 2 leads that are each 1 phono socket to 2 phono plugs, plug the SRL output of the DVD/SACD player into the socket on the leads, and plug the plugs into the SRL and SBL inputs on the amp, repeat for the SRR/SBR.
I also started a related thread "Post processing of DVD-A/ SACD? and which Amp? " here with 50 views so far and no response.
In effect I was looking to see how to have the option of doing 5.1 DVD-A/ SACD into 7.1 (DVD-A/ SACD media is only available in 5.1 max)
Although I am still exploring I have found out that it is all down to your amps capabilities. There should be some info in the amps manual that will explain what inputs the amp will allow 'post processing' on and what it won't (and for that matter what post processing).
So far I understand that both DPLIIx and THX can be used to create 7.1 from a 5.1 source. This can be done typically via i-link or potentially the 6 coax leads (yet to find an amp that will do anything other than bass management though!), but depends on the direct vs processing path options of the amp.
There is also the question - will it sound any better in 7.1 vs how it was mastered in 5.1?
It would at least be nice to have the facility to experiment, especially when there is already conflicting advice on rear speaker placement for a 5.1 setup (at the sides vs at the back of sitting area (Dolby, various magazines, retail outlets etc.)).
I wonder how many people are hearing DVD-A/ SACD the way it was meant to in the first place!