Get a pair of speaker-to-line-level adapters from an auto audio shop or Crutchfield.
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All you need is four resistors. A subwoofer amp for which I have a schematic shows the Hot speaker signal loaded by 100 ohms 2 watts to ground, and a 330K 1/4 watt resistor from the speaker signal to the low level input of the amp. In this subwoofer amp, the speaker signal is also taken through a 150 mfd capacitor to a speaker (4 or 8 ohms) which you will not be doing, so I would suggest that you use a 10 watt resistor instead nof the 2 watt item.
I recently experimented with driving low efficiency (dynaudio) speakers from a plasma display audio amp.
Here is the report.
The plasma display is Panasonic TH-42PWD6. This is a monitor without a tuner, but it does apparently include the audio amp, although speakers are an option that I did not buy. The audio amp is rated at 8 watts per channel.
I wired up two Dynaudio Gemini speakers, low efficiency but good sonics. They are rated at 4 ohms, but measure about 3.5 ohms dc.
For a signal I played CDs on my Pioneer DV45A player: stereo outputs. This is a fixed level output, but the plasma has volume control.
Using the signal directly from the DVD player the volume could be cranked up all the way, and the music was only moderately loud. Sonic quality was not stunning, but OK, certainly for TV or movies (explosions excepted).
To see if I could get more volume I hooked in a preamp to provide some gain between the DVD player and the plasma. This did permit increased volume, but not much before the onset of distortion.
With direct input from the DVD player a "loud" recording registered 78 to 85 dB on the trusty RS meter.
With the additional gain of the preamp the same music was 84 to 91 dB. Louder resulted in distortion.
If high efficiency speakers were used adequate volume would be achieved for applications other than pure music listening. (and movie LFE).
Direct feed from DVD and/or Direct TV receiver is recommended. (Extra gain is of little advantage).
A small subwoofer would be a good idea. Drive it from the speaker signals so that it responds to the volume control of the plasma.
At the present time, put your money into decent speakers that you won't need to upgrade anytime soon. Drive them with the little 8 watt amps.
Next, (when you save up the money) get a powered subwoofer good enough to live with later on.
Next, get a Panasonic SA-XR25 or 45 or whatever is going for about $300 when the time comes. (Now you will get full performance out of those speakers).
Next, get three more speakers for the front, and move the others to the rear.
Next, remortgage the house, and get into high end.