I'd consider something that is both a DAC and amp in one, as if you buy an amp, it will just amplify the crap sound your laptop makes. If you're not acronym familiar, "DAC" stands for "Digital to Analogue Converter". The quality of the DAC is very important when it comes to the quality of the music you hear. The DAC built into laptops these days are always rubbish.
Headphones are basically very small speakers. The reason amps are important is that for a good sound, they need to be able to constantly and effectively send rapidly changing voltages to the headphones. In the case of your HD-650s, it's not unusual for people to buy amplifiers costing thousands of dollars to use them with, as the quality of the music they are capable of delivering scales that high.
Since a DAC and an amp are separate things, you can get them from many manufacturers either separately or in one box. Separately you have the convenience of changing one or the other. Together you have convenience and simplicity.