For the HD600 and looking for a DAC/Amp to match, I'd say you are looking at $250 for DAC and about 300-400 for the Amp.
Audio-gd DAC-100 for the DAC - DAC
HeedAudio CanAmp for the Amp - $400, Audio-gd C2C ($335 Promotion, current best bang for the dollar, CAST Krell Technology!), Purity Audio KICAS ($325), Graham Slee Solo ($400+)
For Combos you are looking around $500.
Audio-gd Compass currently has promotion for $258, a steal. Get the DAC-100 DAC and a discrete Amp that can compete with other 300+ discrete solid states.
There are tubes also, which will cost you less if you go with Little Dot. But other Tubes like Woo/Dark Voice are bit more expensive. In general, you can get more for the dollar with solid state than tubes, especially after buying more tubes for the Amp.
LD MK III or LD MK IV SE with the DAC-100, that will work great for you!
Around $400-500 for that combo. Is it worth it, only your ears can decide that, NO ONE here at Head-Fi can tell you if it's worth the price for you.
Side Note: ANYONE that recommends the HD555 over the AD700 has not heard the AD700.