Connecting a subwoofer.....

There is, in theory, no difference between pre-outs and line-outs, so I cannot understand your question. You need to use a volume-controlled output and, if you are using a stereo source, you need to combine the two channels for the sub. Many subs actually have L and R inputs, thereby eliminating the need for a Y-connection.

So, be much more specific about what you have and what you want and, perhaps, we can help.

Kal

It sounds like you are talking about connecting the sub to a power amp or an integrated amp.

Ideally you want a sub out. Rel subs have a high level input that taps off the speaker connection right at the amp (or integrated). There may other sub manufacturers that have a similar high level input.

Xt16

Yes, I am familiar with that but I am not aware of other subs with Hi Level out. The REL seems to me the only ones I can use with a Naim Nait 5i that has no pre-out. To hook other subs up to this amp isn't possible apparently because of the lack of pre-out.

KR4,

I just mentioned the "Y" splitter as an example of possibly damaging something by messing around with this and not knowing what I am doing.That is, I don't want to make any other mistakes and regret it due to trial and error.
The question was basically asking is there any benefit/disadvantages from using either line level or pre-out when hooking up a sub that doesn't have high level outs.