What are you cleaning? If the Purple Power contains sodium hydroxide it will attack non-ferrous metals like aluminum and zinc. The ultrasound seems to amplify the action of anything you add to the water. I notice if the solution gets old it starts to leave a "soap scum" on the parts if they are in the machine too long or aren't rinsed soon enough.
I usually use LA's Awesome in mine. When cleaning something more delicate I use Dawn and water. I have added quite a bit of Dawn and never had problems but it also didnt seem to help. The heat seems to help the most, I can tell when I forget to turn it on
On a serious note...I would think that an extra amount of soap/solvent would be overkill, since the sound waves, and heat does most of the work. (as Can't drive 55 stated). We always use a measured amount in ours. Some cleaners I have seen (such as ones for electrical parts, and plastic), just use water.