These are a few advantages (some real and some claimed) to using a Y adapter:
- If you leave one of the RCA inputs open, dirt may get in there.
- Using a Y adapter may add a few dbs to the subwoofers's output.
- Some claim that the subwoofer works better at lower levels with the Y-cable.
- The auto-off mode of some older subwoofers (e.g, Velodyne CH-12) may not come on at lower levels without the use of a Y-cable.
- Some older subwoofers may actually need a Y cable because the left channel L may not default to mono signal.
- Another benefit of using a Y-cable at the sub input jacks may be for reducing the amount of Radio Frequency (RF) and Electomagnetic (EM) interference, because you are not leaving the other input open and active. Subwoofers can be affected by annoying buzzes and hums.
Conclusion: Since Y adapters cost a few dollars and there is no significant risk involved, I would recommend using a Y adapter (2 male, 1 female) at the subwoofer's RCA input jacks. Make sure you use a good quality Y adapter that is shielded. Otherwise, you may hear hum and noises.