Headphone amp?

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.

if you really want high end headphone and headphone amp answers, here is the best forum to look at and ask your questions. a few here on Audiogon know stuff, but this is where there is thread after thread of information to read. which might help you to know the right questions to ask. and it’s up to date and cutting edge.

also Classified on this site is very active. and....drum roll.....free.

https://www.head-fi.org/forums/high-end-audio-forum.113/

I own Sennheiser HD800s. You can see my system under my user ID. I also have Focal Utopia, LCD, Beyerdynamic, UltraSonde… and other headphones. I listen to the Sennheiser the most, although I would have to give a slight sound advantage to the Focal… but the Sennheiser are lighter, comfortable with a much more open sound stage (open backed is why)

Headphones and associated gear are just like every other piece of high end gear. You need to support them with the best possible source and amplification. You want them to sound good, you need a very high quality and warm headphone amp. Given your main system amp, I can make some assumptions about your taste. The amp helps warm up and render your system more natural sounding. The Sennheiser need ample power with a very natural sound. I highly recommend Woo head amps. Woo knows how to build powerful natural sounding amps. I have searched for years through different amps… imported one from Germany that was real powerful. I found Woo to consistently provide the best natural sound.

I have used a WA6 (~$1K), WA6SE (~1.5K), and finally now a WA5 (~$5K). Each step up made a jump in performance, that last by far the biggest. The 800s are really addictive with massive 800B tube power. Woo makes different lines as well. I highly recommended any of his stuff. Quality and outstanding sound. Of the caliber you Pass.