sub connection

I connected an old advent QRTZ 10 Subwoofer to a Sunfire TGAIII with a single monster400rca sub cable
even tho the sub has left and right RCA inputs
the preamp only a single RCA out,

I was going to get a Y jack, ( 2 male to 1 female)
but with only a single speaker in the sub, are the right and left rcas internally joined, so a Y would be no use?and a single cable connected to either rt of left rca do the same thing.?

thanks