Your real problem is that Amplifiers are a consumer item.
If you stick to plug packs (wall warts) you are free of electrical licensing as you know, so I think you are on the right track there. I bought a rather large 55 watt 18VAC 2.2Amp plug pack recently for a radio application that cost me less than $20 at Jameco. However it is a US only voltage. So I suspect they must be out there.
Maybe something like
http://www.jaycar.com.au/productView.asp?ID=MP3045&form=CAT2&SUBCATID=1000#12If you stick to designing test equipment, R&D equipment, unfinished products, boards and kits you are generally also exempt of EMC/FCC requirements. ie all of the kits, boards and modules you see people selling all over the net without any regulatory hurdles. If it were me, and I wanted to do a consumer item, as I said above I would probably do it in Asia.
I'm digging into some long lost memory here but I thought that one way around plugging into the mains was to use an isolation transformer. Something like the following
http://www.flanagan.com.au/isolation.htmTechnically then you are not plugging into the mains. I wouldn't run the idea past the electrical board because their standard answer is no but my guess is that you wouldn't find anyone who could tell you that was legal or not - but at least if they did find out and crack a fat about it you could demonstrate that you had a good dig to do the right thing. Again, its easier to ask for forgiveness.