DntMessWitRohanHigh gain mode on the DACport Slim will be miles ahead of the heapdhone jack on your computer/phone, but these drivers are known to scale particularly well with more powerful amplifiers. It's all relative. I can say you'll have a good time with the DACport Slim, but more powerful/different amps will present noticeable differences.
DntMessWitRohanTo add to what @Will said, my DACport slim does a respectable job with my HD600 on high gain (for a USB dongle). But it sounds noticeably compressed and thin in comparison to driving them with the NFB15. The Slim will do well for the HD6XX compared to your PC headphone jack or phone, but the DACport slim is really designed for IEMs/portable headphones and sensitive full size headphones only. I'd recommend a proper desktop headphone amplifier for the HD6XX.