Heya,
Absolutely. I have the LDMK3 and these headphones, and I absolutely plug my 500ohm resistor up when using it with my low impedance headphones (like my Denons for example). The LDMK3 outputs only like 100mW to low impedance loads (32ohm and under), but more than triples for it's intended range, which is 300~600ohm loads. It outputs 350mW at 300/600ohms each. It's literally made for high impedance. This is normal for an OTL amp that is all tube, and not using solid state buffers. They are all meant for higher impedance in general. So the LDMK3 peaks it's performance at 300ohms and 600ohms. It does 300mW at 120ohms, so it can at least handle the 150ish ohm weird headphones out there. But in general, everything put on this amp should be of relatively high impedance.
I use a 500ohm resistor, becuase anything under 100ohms to me is low impedance, and this brings it into the near 600ohm range, a peak range for the LDMK3.
My resistor is a single little 6 inch cable with male/female terminals and a 500ohm resistor soldered inline to each positive lead. It was made by Brian at BTG Audio. It was not expensive at all. The resistors are $1 from parts express.
By the way, the LDMK3 has a gain switch inside. Goes up to 10x. I would actually check to see what it's set to, and drop it down. No need for 10x gain. Clean up the sound, and flex it's muscles.
Very best,