Don't know of any offboard decoders of this kind - you'd need somethign able to take LPCM and convert it to multichannel analogue, assuming the 'older amps' have multichannel analogue inputs.
I suspect that if there were such devices they'd end up costing almost as much as an entry-level receiver able to handle the HD audio from the PS3 as LPCM, given that you can buy such receivers for under £250 these days.