You are correct. DD, Phoenix gold old school, ppi old school make a 5 channel amp that put right around 900-1krms to the sub channel. Also the sundown sae 1100.5 does at least 800wrms on the sub channel, they always underrate their stuff so I bet it does at least 850-900rms. We will have to wait until someone dynos it.
Also audiotec fischer make some high end amps that do around 700wrms or more to the 5 channel models.