Well, first remember I have never used an Acom SS amp ...
If you look at Acom's website, they do have both an owners manual and a CAT document. They imply that they intend for you to use the CAT port (15 pins) for this, which would require a custom cable to adapt the three RS232 signals (RX/TX/GND) back to the 9 pin connector on your USB RS232 cable. This is similar to what SPE does. If you do try this, you may have to invert the RX and TX connections (or use your null modem adapter) if it is backwards.
Since you are using the RS232 port, I don't think the amps setting in the CAT area are what you want. Are there settings for the RS232 port? You are not currently trying to use the CAT port, but what they call the "Remote RS232" port.
www.acom-bg.com/products/amplifiers/acom-1200s-solid-state-18-54-mhz-linear-amplifier
I know that Val at Acom is very good at customer service. You should probably email them and ask if the remote port can be used the way you are trying to, or if you have to use the CAT port and custom cable lash up. I think the main reason they have the 9 pin RS232 port is to connect their remote control head.