I haven't heard the D7100 - but I'm actually wondering if the LD MKIV is actually a good match for them. The D7100 has an impedance of 25 ohms, whilst I think the LD output impedance is more in the 40-50 ohm bracket (from memory). it's an OTL anyway - so the high output impedance is going to better suit headphones with a higher impedance. What I'm suggesting is that due to an incorrect damping factor, you may find that the LD MKIV (or any OTL tube amp) could end up playing havoc with the D7100's frequency response due to poor damping.
You may be far better trying a hybrid tube amp with a lower output impedance.
If you wanted to go Little Dot - you could get the DAC_II (which would give you DAC + solid state amp), and then add a hybrid like the LD MK1+.
Just a thought ......