The DTX790K is a kit configuration that uses the pad set and rack from the DTX900K with the DTX700 drum trigger module. The DTX700 has preset trigger setups for the DTX700K and DTX750K. To get the maximum expression from the DTX-PADS you should create a User Trigger Setup.
Most electronic drum kits use a standard configuration: kick, snare, three toms, hi-hat, crash, and ride. With Yamaha DTX drums there are many ways to upgrade and expand your kit, from adding an extra tom or cymbal, to increasing the size of a pad.