Does anyone own this combination (or similarly an 150 ohm set of headphones while using on board sound chip)?  If so, what was the experience like?
 
I spoke with an EVGA rep who said the X99 motherboard supports up to 600 ohms, but I have not been able to find any posts from someone who owns these headphones and can confirm they did not need to buy a sound card for optimal experience.  I did see some amazon reviews from 2015 mentioning that using on board chips in general did not allow these headphones to work well (IE white noise, cutting out sound, etc).
 
Any feedback is appreciated, thanks!

I have an ATH-M50 (38 Ohm) and the X99 Classified had difficulty driving them enough to satisfy my wants. I don't listen to extremely loud music, but I do like to turn it up a bit sometimes.
 
At 100% volume, the X99 Classified couldn't drive them enough - and with a really old USB Sound Blaster X-Fi Surround 5.1 Pro, it was easily three times louder (or more). I couldn't turn it up to 50% volume without worrying about hearing loss.
 
If my X99 Classy couldn't drive a set of 38 Ohm cans enough, I really, REALLY doubt it can drive a set of 150 Ohm cans properly. Although it could just be that my X99 Classy has a bum built-in sound card..

I recommend sound card to everyone. 
EVGA X99 Classified have perfect small x4 PCI-E slot for Sound Blaster ZxR.
I didn't had bad sound card before, Sound Blaster Z 4 years, and when I replace with ZxR before few months I still feel more quality sound.
And always remind me how much is important best possible sound card. Difference is huge and it's excellent when your nice hardware produce and premium sound. Some people buy nice PCI-E sound card and use only 2 small but expensive speakers.
There is small speakers as Q Acoustics Concept 20 (White or Black) perfect to stay near
monitor and look beautifull but cost few hundreds and sound is incredible.
Immediately you start to love your PC much more than with onboard sound and cheap speakers.
From other side some customers like to connect to pair of Speakers + Center + Subwoofer.
Left and Right + Center + Subwoofer for same money as 5.1 system sound is more quality.
Sometimes Subwoofer is not necessary. Other people like headphones, but again Sound Card is important.

So follow up: the headphones work amazingly with 5.1 when plugged directly into the board.  Nice and loud at about 50%, great bass.  I can't crank the volume to 100% because it's too loud (a good thing).  
 
However, when I try to use the 7.1 adapter, it loses it's strength in volume and sounds more faded.  Also, sometimes the sounds are a little crackly sounding.  Will probably just stick with 5.1 for now until I can troubleshoot a bit more.  

GTXJackBauer

drmarc
So follow up: the headphones work amazingly with 5.1 when plugged directly into the board.  Nice and loud at about 50%, great bass.  I can't crank the volume to 100% because it's too loud (a good thing).  
 
However, when I try to use the 7.1 adapter, it loses it's strength in volume and sounds more faded.  Also, sometimes the sounds are a little crackly sounding.  Will probably just stick with 5.1 for now until I can troubleshoot a bit more.  

It's 7.1 so its going to sound less than say 5.1 or stereo.  I can be on the same volume setting and once I switch to stereo from 7.1, it gets very loud as opposed to 7.1.  I think this is the reason why I've been looking for possibly a 3rd party sound card like the ones I've posted in my first response.  It takes time getting used too as I love 7.1.

Ah I didn't know this.  Thank you for the information.  I will see what I can do about a possible future sound card purchase.  The SLI configuration I use is probably not going to allow me to slip the card in, unless it it's height is short enough that the SLI bridge can go over it without touching.

I'm using an outboard tube amplifier to run my headphones through ...
I find the sound to be warmer and richer that way
 

 
running usb output to usb input of a XDUOO TA-O3 tube amplifier
https://www.massdrop.com/buy/xduoo-ta-03-dac-amp
 
I have upgraded the preamp tube to mullard ... have a couple more I need to try and decide which I like best 

 
headphones are a set of massdrop AKG K7XX
https://www.massdrop.com/...x-akg-k7xx-red-edition
 

GTXJackBauer
Wow.  That's some amazing hifi audiophile gear you got there.  My assumption is, you can only use stereo mode right?  
 
Also, I always wondered about the whole changing the tubes reasoning.  I know you can get different sound from different ones but by how much is the question.  I think you'll spend some good cash to find the sound you personally prefer when all is said and done. 
 

thanks, I'm a bit of an audiophile, and yes, stereo mode only with that setup
If you look at pricing on massdrop and other places, it really isn't all that much compared to some of the other bits I paid for my current rig (and a few other purchases I have planned in the next few weeks)
 
the stock tube really wasn't all that bad, but I like to try out different things ...
as far as the other tubes, I have them already (lucky enough to have a few very good friends who have donated those)