MartinLogan source muddied in low mids....

I have had my ML sources for about 6mo. Overall I am supremely happy with them. They provide more balanced sound than I expected them to. The one issue i keep running into is a bit of confused or muddied sound in mid to deep male voices. They seem to have significant roll off. Is this a failing in the speaker, my interconnects, or amp? any help?

Amp: Pioneer elite sc-25
Interconnect: Audioquest sidewinder
CD player: Marantz cd5004
Speaker cable: Audioquest type 4
Speaker: MartinLogan Source
This is just my opinion, but a "receiver" that uses a Class D ICE amplifier circuit can't be too good. I would try to borrow a seperate 2 channel power amplifier and see how it sounds.
I would try an amp that could drive 4 ohms also.The speakers are 5 ohm nominal,with a dip down to 1.6 ohms if I'm reading the correct manual.Your receiver is rated down to 6 ohms.I think trying an amp is a good idea if you could get your hands on one.Links for both.[]Page 25 on back of receiver.[]
One other possibility that may be causing a problem is the automatic equalizer,if your using it.It may be trying to compensate for a slight room problem,and over/under driving
your ML's at a certain frequency.I've read where some reviewers state that they had to fine tune it themselves. I don't have enough hands on with them,to give any advice for them.
Thanks for the advice guys, I borrowed my buddy's Marantz integrated and it knocked the pioneer out of the park. The issue with muddied mids was resolved simply by switching amps. I had hoped my existing receiver would be sufficient to push my newer speakers but it seems a dedicated 2 channel amp was a far superior option. I appreciate your input.