The 40 Watt Power Amp's Data shows 40 Watts Max into a 4 Ohm Load.
If this Power Amp drives an 8 Ohm Load, the Max will be 20 Watts.
Now, if this Amp can continually push that 20 Watts into the Woofer (largest Speaker) via a Digital'ish signal, that will be quite thumpy!
Unless you really want "noticeable" Low Frequency response, the 20 Watts per Channel - 2 Channel Output from this Power Amp should be fine.
As mentioned by Greg, the RMS "Actual" (continuous) Power Output for this Amp is 8.0 Watts into a 4 Ohm Load - which loosely translates to 4 Watts into an 8 Ohm load.
From the looks of the Amp's design, I would say this RMS Power would find it's way into the Mid-Range and Tweeter Drivers most of the time - with the Transient Max Power pulsing the Woofer for very short bursts.
Hopefully the High End (Tweeter) Driver has a proper Crossover!
Power Amps with low output Power ratings tend to fry Tweeters (Tweeters resonate "easier" with very simple Crossovers and low power amps).
Cheap-'o Capacitors exacerbate the issue - until the Cap pops!
For fun... to "clean up" the signal leaving the Sound Card, Pre-Amp the signal with a 10 Octave Graphic Equalizer.
Bypass the E.Q.s on the PC (i.e.: Windows Media Player's Equalizer) by placing all the slide pots at the center "0" position.
Only Equalize the signal through ONE EQ! Things begin to sound very crappy when Two or more EQ's are working on the same output signal!
This just in...
I just noticed the power Amplifier you obtained is rated for 75 Watts into a 4 Ohm Load.
This should give you around 38 Watts Peak into your 8 Ohm Loads (Speakers), with a continuous RMS Power of around 9 Watts. Again, these are "Per Channel" ratings.
There are many different Tweaks which may be applied to an Audio System... some are intense (read: lotsa' $$$), others are quite modest (read: realistic, with good quality sound, not lotsa' $$$).
Good luck with your Digital Audio Quest!
-- Scott
