I've personally ordered the "Lovely Cube" - quite similar to the m-stage - for my hd650's. After reading through the appropriate threads for both amps, the consensus is that they're both truly excellent SS amplifiers for the asking price, and can generally drive almost any headphone wonderfully, including the hd598's. You can also change the sound signature with different opamp selections and class A biasing if the default setup isn't to your liking, or even remove the input caps to reduce the "warmth" this amp supposedly has. If you're willing to spend $225-250, it sounds like they're hard to beat.

If you only need an amp, I would personally go with either of the above. If you need an amp and dac, the fiio 9 is apparently quite hard to beat on a budget.

As for the NJC Monitor 2, I've heard very little about it. I do know it's quite powerful, very neutral, and supposedly compares favorably to the Solo SRGII. Here's a few reviews if you're interested:

-  http://www.avforums.com/forums/headphones-headphone-amps/1307612-monitor-ii-headphone-amp-review.html

-  http://theartofsound.net/forum/showthread.php?t=10950

I never had much luck with my Matrix M-Stage, I just had one problem after another with it , first one arrived with the left channel dead, second one seemed ok at first but then developed a buzzing in one channel so I sent it back incase it done any damage to my headphones, then the final third attempt looked like I may have lucked out, everything seemed ok at first, apart from the volume control which was sticky to turn, but then one night I switched on & loud pop from inside followed by a horrible stink, I hadnt plugged my headphones in at that stage so I counted my lucky stars.

So now I am finished with that, I gave it a fair crack but after 3 attempts never again, I am now looking for another make, after the original poster mentioned the NJC amp I have been looking at that, seems to be getting great comments so I might just bite the bullet & get one of those on the next pay day, it has got to be more reliable than my recent experience, I could have just been unlucky I guess but 3 times? once bitten twice shy is that old saying, well I was bitten 3 times