Where's ampfixer? 
 
Don't laugh (too loud). Here's my next hobby project:
 
I have a cheapo Logitech powered bass woofer and want to take the amp from it and power a better subwoofer that came with my Sony Home Theater System.
The Sony speaker is not self-powered but sounds pretty good through the Sony Home Theater amp. 
So I thought I would gut the Logitech and rig it to the Sony.
I have no idea of impedance matching or amp power, etc. 
I don't want to use the Sony amp at all. 
Thought I would give it a go. 
 
Any thoughts? I promise not to electrocute myself.
 
Thanks.
 
Rimshot

It would sound a lot better through the Sony. I doubt the Logitech amp would be able to push it very much before it started distorting.

Thanks for that Leadfoot. That is what I need to check first.

The impedance is rather irrelevant as far as getting it to work, speaker amps don't actually have an "output" impedance, they are just able to supply different amounts of power based on the woofer impedance.   Like was mentioned, it is likely the logitech amp is under powered for a good speaker and you won't be able to drive it very hard.  A good class D amp for subwoofer use is at least 200 watts.  Most computer subwoofers that I has seen are about 25-45 watts.

Hey guys.  I did it!
 
The Logitech powered woofer was 30 watts. I open it up and spliced the speaker leads with a long cable coming out of the bass port. Wired it up to the Sony woofer which is rated at 60 watts.  Voila! Sounds great. 
I am in a bedroom size room with near fields. Just needed a touch below 55 Hz and this does it. 
Tied the Volume control of the Logitech amp line input to the stereo miniplug out on my NanoPatch. 
I then adjusted my main powered speakers so that there is a good balance getween them and the Sony subwoofer. 
I have a preamp volume control also on just the sub.
This worked out great for me.  Wish I had a pro rig for this but it's fun and rewarding to make some old junk turn into something fairly usable!
 
I will probably tell everyone on the song forum they are bass heavy now!

 

Good job getting it working Rimshot! Glad it's working for what you needed it for. Can't beat the price!

Watch for heat on the amp when used for long periods of time and loud.

Thanks Leadfoot!
 
Grem, since I am using the same powered woofer but only have connected a different speaker, what that make the amp heat up any more than normal? 

Interesting topic as I just scored a Yamaha YST SW45 Powered Sub for $19 at Goodwill today. I bought it because it was Yamaha and I'm a Yamaha sort of guy. I see they go on E bay for $ 24 and up.
http://usa.yamaha.com/products/audio-visual/speaker-systems/subwoofers/yst-sw45_black__u/
 
 I downloaded the manual and that's what made me realize that you got lucky. The Yamaha, like a lot of powered Subs and Class D type powered speakers uses Negative feedback system to optimize the lousy 70 Watts of power and make it sound more like 100 watts. So just tapping into the power amp is not always an option I would gander. Al lot of these things are complete systems in a box with DPS processing etc. 
 
Anyhow I fired it up and it works. It has a hi pass control that lets you select from 50 to 150 Hz as the cut off. At 50 all you get is a nice thump, at 150 it gets cluttered and you have to turn it down. Anyhow it's dead quiet which my Altec Sub wasn't it also has auto stand by which is perfect for the way I'll use it.