Try an Emotiva XPA-2 or even two of their monoblocks (XPA-1).
You can get them brand new for the price of the used amps mentioned above and they are very solidly built.
I currently have an XPA-2 pumping my 805S, probably a little overkill, but these would drive your 804S beautifully.
XPA-2 - two channel 300w @ 8ohm & 500w @ 4ohm
XPA-1 - monoblock 500w @ 8ohm & 1000w @ 4ohm
You can get them brand new for the price of the used amps mentioned above and they are very solidly built.
I currently have an XPA-2 pumping my 805S, probably a little overkill, but these would drive your 804S beautifully.
XPA-2 - two channel 300w @ 8ohm & 500w @ 4ohm
XPA-1 - monoblock 500w @ 8ohm & 1000w @ 4ohm