high pass filter

As Kr4 said, a cap will do the trick BUT you will need to know the impedence of your bookshelf speakers at 80hz and whether you want a shallow rolloff(6db per octave) or a sharper rolloff(12 or 18db per octave).

Also, the quality of the main cap will make a BIG difference in mid and upper octave transparancy but a plain non polorized electrolytic will also do the job.

It would help if we knew the brand/model of your bookshelf speakers...

Cheers

Let's make this easy, for a ~4 ohm impedence at or around 80hz, you will need the following:

6db - 500uF cap
12db - 350uF cap, 11mH shunt coil

For ~8 ohm impedence:
6db - 250uF cap
12db - 175uF cap, 22mH shunt coil

These values will get you in the ballpark and most likely be just fine and fine tuning the values in relation to the exact impedence at 80hz will yield better results but in most likelyhood, your room acoustics will swamp any differences an exact impedence match would reveal.

Use ~100 volt non-polorized caps in the main feed and build an external crossover box...

Cheers

Hey Duke, I was just going to suggest that the OP consider using a line level x-over instead of the high level route which at the 80hz frequency can be rather daunting.

Harrison Labs makes two such line level x-overs called the FMOD or the PFMOD and will be much cheaper although the quality of their line level x-over's leaves a little to be desired. Ask me how I know... :-)

http://www.hlabs.com/products/crossover/

Cheers