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.?

No need unless you find that the AutoON function (if your sub has that) is less responsive than you require.