Help me complete a system - Gaps here

I've been replacing an older (20 years +) B&K system with new components that I really like, but now I've got some good pieces with gaps in between.

Would appreciate the thoughts of this group on what to do.

The heart of my new system is the NAD M2 Integrated Direct Digital Amp. It works as a very clean DAC and Preamp / Amp and I really like the sound with my Revel F32 speakers.

Unfortunately, my wife watches TV in adjacent room so not enamored of my playing music at ample volume.

Excuse to get a good set of headphones!

I've figured out that the NAD M2 is digital output except for the speakers, which are analog. So requires a headphone amp which is also a DAC.

I've bought the HiFiMan HE 500 phones, which have excellent reviews, but supposedly require a fair amount of power.

I like the headphones a lot, but they didn't sound quite as rich and full as I had expected so I wonder if the headphone amp really has enough power; it is a NuForce Ikon HDP headphone amp.

Any suggestions to bridge the gap between the digital output of the NAD M2 and the headphone amp that won't break the bank? I was okay with the $450 NuForce, and can return it in next 10 days for refund, so probably could justify (to myself) $750 or so total cost for an improvement. But it should be significant improvement.

I'd try a Benchmark DAC1. Excellent DAC, superb headphone amp. They run $600-650 used. If it's not for you, no problem selling it on.
"What HiFi" has a review/comparison of a pile of cans. That's right...cans.
Let me get this straight: you just bought a $6,000 amp/DAC combo, and you want to use it to listen to headphones without using the NAD's amp section or DAC? Let's get creative here. Why not get your wife some wireless headphones for watching TV instead : )

If that doesn't work, I'm wondering if there is a mechanism for hooking up phones to the speaker outputs, so at least you are utilizing the NAD's DAC. If I understand this device correctly, it does just that: