Hello!
I am experiencing a strange problem: I bought a Tascam US2x2 a few weeks ago and installed it properly according to the handbook.
When I use the interface for monitoring, the bass drum sample of the MT Power Drum Kit VST Plugin becomes nearly inaudible. And the monitoring during recording is very quiet. Also, the recorded track shows very low output. All of these problems disappear when I use the onboard soundcard for playback.
I already wrote to the Tascam support, but they couldn't help me.
I suppose it is some kind of driver issue??
Thanks in advance for your help,
Florian

Does the US2x2 have a software mixer?  Check your levels there first.  

The levels are all full up.
 
Could it be an issue with the USB3 ports? I read that some people experience problems with the US2x2 when using USB3 ports. The problem is, my laptop doesn't have any free USB2 ports.

tirkfl
The levels are all full up.
 
Could it be an issue with the USB3 ports? I read that some people experience problems with the US2x2 when using USB3 ports. The problem is, my laptop doesn't have any free USB2 ports.

Yes, that's true. USB3 is the usual "culprit" with these problems, but you would normally see connection-related symptoms like dropouts or noise.  Since your issues seem to only be about low volumes, it doesn't seem likely this is USB3-related. 
 
Forgot to mention this before -- make sure you are using the latest firmware and drivers from Tascam's site.
 
You could have a defective unit.  Is there another PC you could test it on?

It was indeed the USB3 port! I have one single USB2 port on my laptop, where usually the mouse receiver is plugged in. I switched mouse and interface, and it worked :)
 
Thanks, Jon for your help!

Glad it turned out to be a simple fix!