Don't believe that the UX2 has a gain adjustment other than what you mentioned.  Once in PF there is an input level slider...but somehow I don;t think that's what you are after.

FWIW...I've found that a lot of different interfaces have a lot of different input impedance values for 'instrument level' input jacks.  Makes a difference.  I get around that by using a DI box between the instrument & the interface.  Basically takes the unbalanced sig from the inst as input & then outputs, (via XLR connector) a balanced, mic level signal.  The UX2 has gain control for mic pre...so you get to have adjustment play time right there.  Some DI boxes even have fixed or variable attenuators (most are subtractive).

Lots of DI choices out there, cheap to not cheap.  If you want to test the waters with something like this, LiveWire, Rolls & ART make good, cheap passive units that can be had for under $30.00 usd.  (I just mentioned this on another thread...huh).  

Just one way to approach the issue.  Someone else will be along directly with other options.