Zs review is contrary to my experience with the gain settings on the DX3 Pro. I havent noticed any difference between the gain settings on any headphone Ive thrown at it aside from volume. I also havent experienced any issues with switching between pre out and line out modes. Mine is connected to a Topping PA3 speaker amplifier, which, in turn, is driving a pair of Micca MB42X speakers. The only way I can see it being a problem is if you do something silly like crank up the volume on your amplifier or powered speakers to some crazy level before switching to line out mode. Personally, my volume on the PA3 never goes past noon.
Im not just being defensive about a product I own either. I only got my DX3 Pro this week and Im still well within the Amazon return window if I found something objectionable about it. So far, it has been a delight. I just wish it had a 6.35mm headphone output.