My C9 does NOT accept 480i line3x from my PS2. In fact, whereas with some other consoles I may stumble across a a specific timing setting that may generate a black screen, for 480i line3x, the C9 actually generates a fancy graphic that says “Invalid Format”, so it must doubly not like it.
I’ve found that 480i Line4x with alternating scanlines cuts down on the flickering a great deal, and the picture is significantly sharper and brighter than sending the signal via passthrough for the C9 to deal with all on its own. Honestly, I may wait and see what the OSSC Pro brings to the table in terms of 480i deinterlacing before moving my PS2 into my setup full-time.