Thanks, I'll come back to this later if I haven't jumped out the window, controller in hand...

A tip that might help troubleshooting now and in the future:
 
TTS-1 has a little green button that looks like a musical note at the bottom of every channel strip. This is an Audition button that triggers internal playback an instrument-appropriate musical phrase. If you hear sound when you click one of these buttons, your audio routing is all good, and you should check that MIDI is getting sent to TTS-1.
 
On the other hand, if you see activity in the TTS-1 channel meter but don't hear anything, the output is not making it to your interface output or your monitoring system is not working. Follow the chain of I/O assignments and physical devices until you find the culprit.
 

I always hear the instrument when I press the preview icon, getting a response while playing the controller is when it becomes hit and miss.

And still dealing with latency. Is the latency slider greyed out because I'm using ASIO?

Yes you adjust ASIO buffers from the interface's control panel.

I don't know if you're familiar with the Presonus AudioboxUSB, but there is no real control panel... the only thing on it is Sample rate (which I have at 44.1khz) and ASIO buffer size (set to 2048). Anywhere else to look at latency? It's so minor, but enough to make it hard to play along with other tracks.

The Presonus ASIO buffer size is to high. Try setting it to 128 or 256 in the Presonus client.

I have the AudioBox1818VSL, and I access its user interface screens by opening a program it installs that runs at Windows start up in the System Tray, at the bottom right corner of your desktop - where the flag and speaker icons live.
 
Hope that helps, 
 
Bob Bone
 

Thanks! I changed the setting but I'll have to wait to get home to check it out.
 
The Audiobox USB control panel literally has only those two items on it to adjust.

 your description of your input sounds fishy. 
Load the TTS-1  test it like you did. 
If you want different instruments you can load them from the mixer. I set up my own "band" and have a different instrument in each of the 16 channels. Drums are in CH 10. You can save this as a pre set for future use.  
Now insert a blank midi track
Set it's input to the keyboard controller Omni. 
Set the tracks output to the TTS-1. 
By setting a channel I get the different instruments. 
Done deal. Record away. Note that input echo has to be highlighted for the midi track. You won't hear anything if it's off. 
 
If this doesn't work then you have an issue with your midi cable or the controllers midi output. 
Other things to try before you jump out the window :) 
Maybe you need to re install the drivers for your audio interface  Go to the web site check the version you have is up to date and re install it. 
There is also a very hi probability you have a bum midi cable. Swap it out. 
Have you run Latency Mon? background processes can cause weirdness on a PC. 
http://www.resplendence.com/latencymon