I have a RME HDSPe AIO card (PCIe) installed that I currently use to drive my headphones. My music production workflow revolves around two pairs of headphones: Denon AH-D5000's, and a pair of Audeze LCD-X's. I love both of them, but for different applications. Since I produce bass heavy EDM, I really like the boomy sound signature of the Denon's when mixing, because it allows me to really dial in my sub bass levels and actually "feel" the sub bass. It's almost like having a subwoofer on your ears. But I also use the LCD-X's for it's superior midrange, sound stage, resolution, and extreme neutrality. I typically use these later on in the mixing / mastering process. My end goal here is to be able to run both concurrently so I can have the best of both worlds. I was thinking about purchasing a simple 1/4'' splitter and running that into the HDSPe AIO headphone breakout, but I'd really like a way to toggle and independently control levels for each headphone so I'm not constantly re-adjusting levels every time I switch cans. So my question is, what solutions are out there? Is there a simple interface I can buy that will take in one 1/4'' input (from my sound card) in to an interface of some kind that will allow me to plug in two 1/4'' inputs and control their volume independently? I basically just want a small splitter that will give me gain control and possibly a way to toggle each input on/off. (The HDSPe provides plenty of amplification so I don't really need a heavy amp, just something very transparent) Any help is HUGELY appreciated.