I was not able to find any technical data on the OTTO switch so I'm not sure what you can use to control it.
Another approach that might work better for you is to use an X10 controller and plug-in module. The
IR7243W controller accepts both RF and IR signals from your remote. If your amplifier uses less than 500W of power, you can find an
AM12 series appliance module with a suitable plug for your country at
http://www.x10europe.com/ and use it to turn your amplifier on and off.
The X10 technology in Europe operates at 433.92 Mhz and apparently can be controlled directly by European versions of the Pronto that support RF.
The MX-700 can also control the IR7243W using IR. Unfortunately, the MX-800 cannot be used to control this using RF; I don't think that the MX-800 is sold in Europe and the North American version operates on a different frequency (418 MHz) and is probably also illegal to use in your country.
The other advantage to using X10 is that you can use the same technology to control your lighting as well.
Hope this helps...
Greg