Agree with palmer and BB4TB. Let me add that, if you use the Y adapter to send L/R channels to your sub, you will need to tell your preamp that you have no subwoofer. Otherwise, in cases where you have a dedicated LFE signal, your sub won't be seeing that. One difference of this arrangement is that you would solely rely on your sub's crossover settings, unless of course you have something like a SMS-1.
IMHO, the single cable from your "sub-out" to [usually] the left input on your sub is the preferred method.