preamp with only 1 set of pre-outs: adding sub....

Like Bdgregory pointed out, you can connect the sub taking the signal from the speaker posts on you amp and connect speaker wire to "the speaker in" input on your sub (Not all subs have this connection). You would leave your speakers connected as usual to the amp, and this has no impact on the load that the amp sees.
Some argue that this is the prefered method of connecting sub (REL subs being one advocate and fitting their subs with speakon high level inputs), whereas others disagree. There are some threads on that topic here on Audiogon if you are interested.

I have a powered sub. I ran my pre-outs from the pre-amp to the high pass filter on the sub and from the outputs on the sub back to my amp and ran my speakers from my amp, using the crossover on my sub.

I connect my sub using high level inputs connected to my power amps. I find it far better. But if you want to use pre-outs, the y connector is the way to go.

A pair y connectors will allow you to connect both to the outputs. Not a purist arrangement but works fine and I doubt anyone can hear the difference.

Depends on the sub. Some have built-in HP filters for your main speaker/amp.

Kal