Your headphones are rated at 300 ohm, this means that you will need to look at something that will drive them. Next thing you need to think about is the built in DAC that your device has, will this give you the sound you are looking for. The reason for that is because a portable AMP can be found at a cheaper price than a comparable DAP (DAC AMP combo). I have DAC's, AMP's and DAP's and have found that some work better than others depending on the headphones I am using it with. My 598 SE is fine connected to my phone or tablet directly but with a little more power i get a cleaner sound. My 1More triple driver over ear will also work but there is a marked difference in sound and volume with a DAP. My LCD 2 Classic Closed back needs a DAP (the only reason I use a DAP and not just a AMP is because I am not fond of my phones built in DAC). With the 6XX you get the classic Sennheiser sound quality and want to make sure that you are pairing it with something that is going to take advantage of that. I would suggest that if there are any people you know that have portables see if they will let you use it. The audio you get will change from product to product and brand to brand. Testing out as many as you can will give you an idea of what sounds better with those headphones based on the sound quality you enjoy. Nothing about this hobby is easy nor cookie cutter, peoples audio tastes can vary widely which is why there are so many different types of headphones and equipment out there. It doesn't have to be a long drawn out process but I would suggest that you try as many parings as possible in order to find the sound you enjoy. Buying quality headphones is but a piece of the puzzle, the rest is figuring out what your audio taste is and finding a DAP to go with it. Sorry this could not have been a clear cut answer but to be honest, anyone that will tell you to choose one thing over another is just basing that on their opinion and tastes, which might not be the same as yours. I have plenty of equipment and headphones sitting on my shelf that I purchased based on other what I have read and others have told me just to find out it is not what I like. Save yourself some money and when you can, sample the things you are interested in before you buy it because this can quickly become a money sinkhole. I hope this helps, if you have any other questions just ask and I will do my best to reply as quickly as possible.