Its a vicious circle. First you get the LCD2, and then you find the performance a bit less rewarding. Then you buy a new amp, and so on... $2000 spent.
Try upgrading your back-end. What DAC are you using? HD600/650 are well-known to respond well to upgrades upstream, so may I suggest you hold on to the HD650 and try a Source/DAC upgrade first? In any case, you are now entering the realm of dimishing returns. $500 would get you a very nice DAC, I reckon.