I have the 990/600. Had it a while. Used an E-9 with it untill I was sure that my receiver's headphone out was better and sent the E-9 back. Got a Maverick TubeMagic D1 and the amp section just sucks. Still like the DAC section though. Got a new receiver, I know, why not just an amp, well this receiver can be turned into a pre-amp or power amp if you chose and is a decidedly mid-fi Yamaha, so not to bad. The hp out on the Yamaha was better than the first receiver, so I was wondering about all the amp talk here as the E-9 seems to be thought of as a good cheap amp.
Well I took the jump and bought a Bottlehead Crack. It is very simple, great instructions, anyone can build it and the support forum is tops. My Beyers' have took off. They sound as good as anything I've ever heard before, but I don't have the money to go hi-fi, or at least what is thought of as hi-fi in most circles. But this Crack is hi-fi and cheap. Just had a price increase but I think you could well justify the extra outlay. About 279 now I think.
Again, this will make the 990/600's sing, you can change tubes and really hear the difference. Your low impeadence phones will not work with the Crack as it is OTL, and I can't say anything about them, but I am sure that you need to match a amp that runs low impeadence headphones for them.
I have read but have no first hand knowlegde that the Crack owns the LD's.