Updated: Nov 10
Well, you may have already realized there is no connection labeled "Subwoofer" on your 50 year old Pioneer you just bought. Subwoofers for home use were very rare, up until the late 1970s. Among other reasons, many speakers of the time could reach quite low in bass. These speakers were usually pretty big. Something a bit unexpected, was that people would realize in years to come would be that small monitor style speakers did as well or better in the midrange and treble as a large speaker. They just needed some help on the bottom end. This trend led to an increase in the subwoofer becoming commonplace in modern home systems.
Ok, let's cut to the chase here. The easiest way to connect a sub to any amplifier, vintage or otherwise, is at speaker level. This means you run speaker wires to the sub from your amp, and another set of wires from the sub to the speakers. Paul McGowan of PS Audio actually favors this method, and we agree. Your sub would need to look something like the one above with speaker style connectors. This is by far the easiest way to do it. And Paul points out that this also has the benefit of impressing the fingerprint of your main amp on the subwoofer.
If you already have a sub that only has a line input, there are a couple of options. It gets a little complicated from here. If your amp or receiver has a "Pre Out" set of jacks, this could be connected to the sub with a line in only. If your amp has links between the "Pre Out" and "Main In" you would need to use a "Y" adapter to allow the internal amp to remain connected, and send sound to your sub. The other thing, and this is going to bug some people, most subs have only a single line input. Just choose one channel and go with it. Everything will be fine. Nothing to see here. Move along.
Yet another alternative is to choose a speaker level to line level converter. We don't recommend these. A customer recently bought a cheap one from Amazon, and blew the channel of a freshly restored Pioneer SX 1050 he just spent $1200 on. Bummer!
Bottom line is the speaker level connection to a sub is much easier to setup with a vintage hifi setup. If your amplifier has a "B" set of speaker connections, you can connect one set to the main speakers, and the other to the sub. You will of course need to set your unit to "A+B". A bonus here is that you can switch off the sub for late night listening.
A final thought. Be very careful with speaker connections to amplifiers. A mistake can cause the magic smoke to escape. This is bad. Make changes with the power off, and double check to be sure.