Connection between FiiO K5 Pro and X1 2nd gen

Hello,

I just purchased the K5 Pro, but I cannot figure out how to properly connect to my FiiO X1 2nd gen Player. The X1 2nd just has a regular 3.5mm jack, so I purchased the Insignia Stereo Audio RCA Cable. I connect the red and white RCA to the back of K5 pro line-in, and the 3.5 mm to the X1 2nd. The K5 pro can produce sounds, however at an extremely low level. I have to use gain level 3 and tune the knob beyond 3 o’clock to get “some” sound, not loud at all. It definitely shouldn’t be like this. I connect my iPad Pro with K5 pro through USB (via a USB type C / USB A adapter), the sound is loud enough with level-1 gain at 10 o’clock. For reference, I’m using Sennheiser HD600 headphone (300 ohms).

So what was I doing wrong?

Thanks guys.