Hope this helps some people....
Got mine today. Have it set for low gain and am using Mullard m8100 tubes. This little amp is powerful. With the volume knob at notch "30" which is the 10 o'clock position it is pushing my HD650s with authority. I was worried that the LD1 wasn't going to drive the 650s after reading all the posts and head-fi....but holy shit, no need to even try the high gain.
Here is a quick list of the headphones I am using with this LD1 amp with the above tubes and low gain, stock op-amp:
Sennheiser HD650's and 518's, Phillips Fidelio X1, AKG K7XX (massdrop), Beyer-DT990 Pro 250ohm, Phillips SHP-9500 (Z-modded, or best i could), ATH-M50, and some Chinese version of fostex, (yah, i wanted to hear wooden cups on closed backs before spending $499, they were like $100 but sound satisfactory, not for me, i like open back mostly).
Using the stock op-amp for now until my ears adjust a bit, i read so much about all the different op-amps you can get that my face hurt. I also have the WE408A's and the RCA 5654's tubes. For now i can tell you the WE408A's produce a well rounded sound with enough bass to get your head bobbin so i recommend you start there if you ordered the LD1 with that upgrade option, you wont be disappointed. The RCA's and Mullards add more warmth or "V-Shape" to it and great support for the bass to sub-bass range. I like my sound to have a wide stage, low end to be authoritative and mids in the right spot and highs to not make my ears bleed. I listen and game, i don't mix/master sound. I have this combined with a Nuforce uDAC3. Anyways, wanted to say something real quick where as this was my first time with better audio gear. Went from a PC sound card to this new setup and i am blown away, from what i see in headfi, the sky is the limit, now don't go deaf!