Subwoofer Question

Sounds like this'll be a really nice setup once it's finished. I use a Technics amp into a pair of 20 dollar (but loud and fairly decent) cabinets with 12" bass drivers, mids and tweeters, and some crossovers. Really nice to have in a dorm.

Is this subwoofer powered or unpowered? ie., does it have its own amp, or is it standalone?

If it's standalone, you pretty much have two options. The first is to just stick it on the second output channel of your amp, if it has one. This may not be a great idea, especially considering the levels wouldn't really be balanced. You'd also probably need a crossover circuit to cut the highs out. Your second option is to get a second amp, independent of the first (with a y adapter coming off the source), and use its EQ to filter it for the lows and set the level. I'd recommend the second over the first- it'll give you much more control over the sound. Plus, if you only have one out output on the amp and you put the sub on one channel, it'll change the overall impedance of the load. This can cause problems with the sound.

Fox is correct concerning a passive (ie unpowered) sub. You'll need a separate cross-over and separate amp.

Most powered subs have both line level as well as speaker level inputs. Line level will have red and white RCA plugs. The speaker level inputs will be some form of binding post. Married to the speaker level input will be speaker level outputs - also some type of binding post.

I assume your elderly (oops I mean vintage) Onkyo does not have a dedicated "pre-out" for the low frequency. So, here's how to connect your amp:

Run speaker wire from the amp's main speaker out binding posts (L & R) to the speaker level inputs on the sub. Then run speaker wire from the sub's speaker level output to your Technics.

The sub should have an adjustable dials to adjust volume level and cross-over frequency. Play with these until it sounds best. The sub will filter out the low frequencies and send only the mids and highs to the Technics.

All done...