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Warning: If you don't really know what you are doing, don't always expect to succeed. In this case: Be prepared to break your DXG. 3 Volts is above the voltage used and may always break the device. I really, really consider this bad advice.
That said, well, it didn't break for you. Of course it worked best with 3 volts. That's due to the operational specifications for your typical MAX232, which are at 5 volts. For 3 Volts there's the MAX3232. For 1.8V, typically a ttl voltage converter is needed. Everything else is a dirty hack, so be prepared to pay the price (a high one in case of a KDX, if you ask me). Purely singular-evidence based advice like this is a bit out of place here, I think. And please, don't follow the suggestion to use no adapter at all. 9 volts differential signals are not what a typical SoC will tolerate. You have been warned.
Also note that RX and TX are two different matters when you go over voltage. If your adapter is tolerant, it might still receive data from the Kindle when it has long fried the receiver on the Kindle by transmitting with too high voltage.
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