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.