Headphone amp?

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.

You can occasionally find HD800s headphones used, but expect to pay ~$1200 for a complete boxed set in excellent condition.  I waited several months for the pair I recently purchased.

I run mine with a Quicksilver tube headphone amp and Moon Audio Black Dragon cables - a setup which, by audiophile standards, doesn't break the bank. For my listening tastes (mostly female vocals), I rarely take the volume knob past 10 o'clock. (If $$ were less of an issue, I would have looked closely at the Pass Labs HPA-1).

The 800s cans sound wonderful, though I find the HD600 series to be slightly more comfortable.  Still, it's a worthwhile trade-off.

As always, YMMV.

Good luck in your quest!

I use Moon Audio Black Dragon headphone cables, too - I've got them for all 3 of my headphone sets. The Quicksilver tube headphone amp is some serious value for money at under $1000.