ultralight
I got a pre-amp, hoping that it would fix the problem but it didn't. I can have the input to the point that it's clipping and the result in sonar is still way too low.

That makes no sense. Something is not right if that is the case. I am stumped! But I wont stop thinking about it. 

All i can think of is:

Try to replace your cables from the audio interface to the computer.

Is the playback volume low compared to a loop dropped in the project? Or are you just seeing a small wave on the trk?

Sounds like you have a hardware problem somewhere. Sonar cannot have a 'input gaining' issue with its coding.
Check your input selects to make sure something is not properly assigned.
Mono? Mono cabling? Stereo? Stereo cabling?
Input echo ON/OFF.
Your Output assigned to Master Bus or direct to audio card........

Also try turning  off auto sens. I just noticed it has a software mixer/control panel. That may need to be set up correctly. 

On page 38 of the user manual of the Quad Capture it has a section on trouble shooting low level audio coming to the computer.

When you play the track what levels are you getting. Is this just a waveform display issue? Click and drag the little header bar between the clip and track panes to scale up the waveform display.

tKx5050

Is this just a waveform display issue?

This is what I was trying to say!!

I use a USB cable between the audio interface and the computer. Again, this audio interface is behaving exactly the same as the last one.
 
A loop that I drop in, or something from a soft synth has a much higher volume than anything I can record with a microphone or guitar (and visually is much higher/taller in the track window).
 
I wish it was just a waveform display issue. It's not. Volume is very low.
 
Input echo can be on or off. No difference.
 
Thanks for the clues, tries. I do appreciate it.
 
 
 
 

Did you try to replace the USB cable? If you use the same cable on both interfaces I would at least try another cable. Also try another USB port. Preferably one on the back, just to rule certain things out.

If the previous audio interface did not use a USB cable, let us know.

I've used lots of different USB cables. Yes the other interface was of the same type--it connected to the laptop via USB.
 
Thanks!