Headphone amp?

I would suggest you seriously consider a good Headphone amp for your Sennheisers . I also have HD800s and know they are a difficult load so a plug into the headphone jack of your amp won't do. I have The Luxman P- 750 u and my friend has a Pass Labs Head Amp and these are superb amps if maybe a little steep for some folk but there are so many out there and you can find one for both your pocket and your sound appreciation.

Would you use an A/V receiver to power your speakers for two channel listening instead of your Pass gear? 

Yes, you need a headphone amp to fully enjoy a good set of headphones, which the Senns are. 

There are a lot of great headphone amps and even the entry level ones are likely to be better than listening from your CD player.  

What's your budget? 

There are a number of headphone aficionados here, but I'd suggest heading over to the head-fi forums for suggestions on what to get and reviews.

 Thanks, gentlemen. I haven’t bought the headphones yet, and they’re around $1500. Might find them used for less. I’ll research the proper amps.

If you do not have a headphone already then, not clear from your post, these headphones and amp interface box will get you state of the art headphone sound (really almost 2-channel like)

(8) RAAL-requisite SR1a | Headphone Reviews and Discussion - Head-Fi.org

Jotunheim R Sold separately to another user, no longer included.

The above quote in the ad is referring to me. I received the JR last night and it worked perfectly. The JR is not as good as an excellent 2-channel. I am not sure if your PASS labs have enough power. You need a min of 100 watts to drive these properly with a stereo amp.

Edit: I just looked up your Pass Labs and it is not enough power for the RAAL SR1a. An entry level amp to listen to these headphones is the $800 JR.

Amplifiers • RAAL-requisite (raalrequisite.com)

I use a KRELL 175XD (Class A) and a CODA #8 to drive the RAAL SR1a. Along with the used JR I got yesterday.