SEAS ER18RNX complimentary smaller driver for surround.

I built some miniDSP triamped speakers for my main front speakers, the mid bass is ER18RNX which I am happy with. Now I am using some little Polk surrounds + center and they sound different in surround material, and sometimes quite annoying.

I need a smaller, maybe 4" or so driver which will sound similar above 100hz (Ill use the same tweeter)

I was thinking of the SEAS CA12RCY which is coated paper instead of paper+reed, but the motor seems like a scaled down version. Does that seem reasonable?

I know it's not going to be exact, but much closer than the little (ported for some reason) polks.