1) Get a balanced headphone amp. Setting aside debates on how beneficial this really is, which depends on a lot of other factors,* at the very least it will be very convenient for your DAC. Just make sure to use balanced cables for your headphone, and this is where it can get insanely expensive. The cheapest I think is the Ibasso, and you can get adapters from them on both ends of the amp (ie, XLR to HR10 for input, and a Hirose to Sennheiser). Here's where expenses can get tricky - a lot of balanced headphone cables can be insanely overpriced, because most people who make them also sell the SE cable at the same overpriced, "audiophile" price levels. If you don't use the Ibasso and have to get a headphone cable with an XLR output, expect to g on eBay and find them for around $100 minimum (maybe a small additional fee for an XLR plug), and some can be around $300 for the non-eBay brands.
2) Doesn't the NAD amp have a tape loop? As long as you use the multiple inputs (not the direct amplifier input that bypasses the preamp circuit), if it has a tape loop, the same signal will be sent out that way.
NOTE : You might find that some dedicated headphone amps might have too much power or gain for IEMs, and low impedance won't correct problems like uneven levels on both channels even at normal listening levels, or one that can go from barely audible to tinnitus-inducing at a twitch of the knob. So even if the amp has a gain switch, make sure you check reviews if it works well with IEMs. I have Meier Cantate and it works great on IEMs on low gain, which is -10db, but if I had to switch to high gain, I have to open the amp cover as there is no switch outside. Wasn't a problem though as I only borrowed that serious IEM (Westone2) and it drives the HD600/650 well enough at 1:00 on the volume knob, no need for high gain really.
*For one, the higher output on the portable balanced amps, whichis higher than most portables but not really spectacular vs desktop amps that cost just a bit more