Is there an easy way for me to create a simple low pass filter and lower the volume for a particular speaker?

Basically for my home setup I've got two small polk bookshelves, and a massive electrovoice speaker from like the 50's. It's got a 12" woofer, and a giant tweeter centered in the woofer. It's in a giant enclosure and has a variable presence knob on the back. It sounds really nice... problem is I don't know what to do with a single speaker in a 2 channel music setup. My thought was to basically turn it into a subwoofer so that I can at least use it for something. Can I just throw a cap or something on it to cut out the higher frequencies? and maybe some resistance somewhere to cut the volume down? (as of now its louder than the 2 other speakers)