I don't want to appear too enthusiastic about using speaker amps instead of headphone amps...
Headphone amps *can* be superior if you can afford the right one. How good the speaker amp sounds after the adapter depends on... the quality of the speaker amp of course!
You can expect the speaker amp to have less finesse than a comparably priced headphone amp (because of tradeoffs required to achieve the extra power), but it will never be lacking in power. People have commented that it takes 'power' to bring out the bass of the HD580. Personally I doubt that it takes any more power than is provided in an entry level headphone amp, but people have reported gains in the bass when switching to higher end, beefier amps. A speaker amp *could* be a shortcut to getting all the power you need, but then I wouldn't know if the improvements they mentioned were really due to a power issue.
However, none of that matters if there is a series resistor between the speaker amp and headphone amp muddying up the sound. (Long story about exactly how it muddies up the sound) The impeder can undo much of that damage. As for building the thing, this really *is* a simple project and you should consider building this yourself. You need:
A soldering iron
Solder
'Helping hands' to hold on to parts while you're soldering
And the parts mentioned in j-curve's thread.
Lots of tutorials available on how to solder. Just look up 'soldering tutorial' or post a question on the DIY forum.
If you still don't want to DIY, you could post a thread either in the DIY forum or the sales forum asking for one. Lots of DIYers around.
But depending on the speaker amp you have, this may be just a temporary waypoint, for you to see how the HD580 sounds out of some halfway decent amplification: do you like what you hear so far? Do you want to improve things further with a dedicated amp? Or do you need to go for another type of sound altogether (different phones)? So this experiment ought not cost you much. Less than $5 in parts if you build this yourself (although buying a soldering iron, etc. for the first time would cost a bit more) or just a bit more in shipping if you want to buy it from someone.