Here is the way I do QSK with my FT-1000MP which is basically the same set up as the 990 as far as qsk is concerned.
I have an Ameritron AL-80b which has an output of up to 900 watts.
In 1974, I built an Electronic TR switch from plans in the ARRL handbook. It consists of nothing more than a 12AU7 tube, a coil,switch, variable capacitor and a simple dc power supply. There are 3 coax jacks on the back.
To connect this, you run the transceiver's main antenna input into the amplifier's antenna input as you normally would. Run the amplifiers output into and back out of the TR switch. Inside the tr switch, we simply tap off the center conductor of the coax, run this through the tube then to the 3rd coax jack on the tr switch which goes to the receiver input of your transceiver.
As you can see, there is NO switching of the RF involved here at all. No worrys of hot switching or dot clipping. The TR switch provides another stage of RF for the receiver too. Keep all leads as short as possible.
When the amplifier is turned off, simply hit the antenna switch on the transceiver to change the rx back to the main antenna input.
In the past, TR switches were known to cause TVI but with cable tv, there is little to be concerned with. It's a great system.