There'll never be such a thing as "zero" (0) latency, but that's being too technical. I think the ideal is to always achieve the "lowest" possible amount of latency when dealing with audio that gets converted from analog to digital, processed through software running on a computer and then back out through another converter to go from digital back to analog and out though headphones or speakers... then through the air distance to your ear/eardrums for you to actually "hear" the sound.

First off, please ensure you're monitoring from the outputs (either Analog 1/2 outs or the Phones out) of your UX2.

Secondly, in Reason or RECORD Audio Preferences, make sure you have the "ASIO UX2" driver.

I'm sending you a screenshot of the KB37 I have at work and the Reason 5 Audio Preference settings. I'm at 128 samples (Buffer Size) with Input Latency of 7ms and Output Latency of 12ms.

When triggering synth and sampler devices in Reason, I don't experience any humanly perceivable latency from the moment my fingers depress a key(s) on the KB37 MIDI controller and the time my ears are hearing the sound of the synth device being triggered.

When playing back a project that has a higher number of tracks, I typically will raise the Buffer Size up to at least 512 samples to provide better playback (so you don't choke the CPU).

But you can't ever get zero (0) latency. I don't know of any analog/digital audio interface that has this ability. You can however, get low latency!