This is only a guess, but possibly there is a small conflict going on between your Presonus 24 and XT.
Where one driver is taking precedence over the other.
 
Make certain both devices are selected in preferences. Myself, I have not run 2 devices so I cant really be too helpful.
I believe the XT uses an ASIO driver as does the Presonus. However I don't believe both devices can exist using 2 drivers at the same time, meaning sonar will only run one driver at a time.
I cant honestly be certain though, as I mentioned I haven't run 2 devices like this.
 
As a test, I would unplug my Presonus and focus on the XT to get it right, if anything it will eliminate another device as the potential problem child. 
 
* Also note: Brundlefly's mention of ports can be critical as some of these devices are not backwards compadible when it comes to USB 2 and USB 3. Check Motu's spec to be sure but using a device that is meant for a USB 2 hub, in a USB 3 will sometimes work but lead to drop outs, random crashes, exc.

* Also note: Brundlefly's mention of ports can be critical as some of these devices are not backwards compadible when it comes to USB 2 and USB 3. Check Motu's spec to be sure but using a device that is meant for a USB 2 hub, in a USB 3 will sometimes work but lead to drop outs, random crashes, exc.
 
Very true, USB 3.0 uses the xHCI controller whereas USB 2.0 uses the EHCI controller on your MOBO. Some boards have different drivers and chipsets on the same board for each. Mine has an Intel EHCI controller which works flawlessly for all my USB 2.0 stuff. The xHCI controller is from VIA and is buggy even with 3.0 devices so I don't use those ports for anything.
 
 
 

There seems to be some confusion here regarding this unit... the MIDI Express XT is a midi interface, not audio, so there's no ASIO driver or spec to worry about. The latest drivers are here (under MOTU MIDI Installer):
 
http://motu.com/download
 
Also, I think it's been mentioned, but make sure you're using a USB2 port (not the blue USB3 ports).

JonD
There seems to be some confusion here regarding this unit... the MIDI Express XT is a midi interface, not audio, so there's no ASIO driver or spec to worry about. The latest drivers are here (under MOTU MIDI Installer):
 
http://motu.com/download
 
Also, I think it's been mentioned, but make sure you're using a USB2 port (not the blue USB3 ports).

your right. Though it doesn't need a driver either / only firmware.
You wanna know something even funnier ? I looked over in my Rack, and I have one .
Its an older one but its a MOTU midi EXPress XT. I don't use it myself, haven't hooked up as I haven't had the need for it.
 
In this case, the issue has to be a connection somewhere or possible the hardware going through the XT not using the presonus' ASIO driver / thus causing latency somewhere.