You don't necessarily need 3.5mm input as RCA is no difference when using a RCA to 3.5mm lead. Looking at speakers with 3.5mm input only, just restricts your choice.
Speakers with RCA input:-
Mackie CR3/4
M Audio AV32/42
Edifier 1280T, R1600T III, R1800T III
Presonus Eris 3.5
Mackie speakers have a selector switch (mentioned in a previous post) which allows you to select which channel the powered speaker is used for, but that really isn't needed as an advantage with RCA input, is that you can swap the left and right plugs around to choose which side the powered speaker goes on.