I considered getting the 8i6 or 18i8 a while ago, but after some research it seemed like the higher-end Scarlett's tend to come with factory faults fairly often. The interface itself is good though, so I recommend you try to get it replaced.
Maybe you should also get a separate headphone amp or a DAC if you're having trouble driving your headphones.
update: sold the 18i20 for a little less than I bought it for and now it seems to be working well for the ebay buyer who bought it (i think they are using it with a mac!). Went with a gen 3 2i2 in the end and it's been painless thus far c: