that depends on your USB audio interface - if it has LINE inputs then yes, you can send the output of the ART Tube MP into the LINE input of the USB interface.

as far as whether you would get a better sound?  maybe, maybe not.  it depends on the sound you're looking for and what the preamps are on the interface you currently have, etc. etc.

I use an Art tube pre Amp and it has a wide range of output options. It has both Hi and Lo impedance outputs. There is also an output attenuator. It will interface with just about anything. So you are 100% OK.
I use the XLR output and set my interface on "mike". Set the interface pad where you would for a live mike and then turn up the output of the Art until your peak light come on when you shout.
FYI
  Using "testing 1-2-3"  is stupid, I use a forceful   " HEY Stupid Wake UP! " This not only establishes your max input gain it will test for both sibilance and popped "P's" ,
 it will also get the audiences, if anything's , attention...

I would look too at maybe some FX plug ins that emulate a tape saturation sound.

SO a digital imatation of analog distortion>>> hmm,, that can't be good..

You are correct regarding that good pre amps are expensive say  $1,000 ?
But for the money the little Arts are usually better than the sad fare found on inexpensive interfaces.
In my case that's a M Audio fast track pro. The Pre amp just sounds dead. Pre amps have to interact with the circuits of your mike. The M Audio pre amp is way below the quality you would find on a mixing desk. The Art on the other hand, is more of a standard issue pre amp so works great.  It is a great little DI box too. Not sure the tube does much but it impresses people when you patch them into it on stage

I also have a Tascam interface where the pre amps are as just fine by themselves. 

Yes, 24 bit will make a difference, esp. for headroom.  You should be able to switch it in SONAR's Option Audio page, if it isn't hardware/UA 25 software controlled.

The Art stuff is OK - if it is a starved plate design you might find it a little noisy w/ vocals.  I have an mp that I use for bass and it sounds good, if fluffy.  The Cake/Roland stuff usually has OK preamps although clean.  That can make a nice combo.  You can plug the ART into the Cake interface via the front inputs.  They should be automatically sensing inputs if it is like the other Cake stuff I have used.  Just plug the ART line out into it and it does all the matching.

As far as a good preamp - they are expensive.  The best buy lately has been the Focusrite ISA One.  They were blowing them out for $500.  80 db of gain, transfomer-coupled input for some iron goodness, and impecable heritage coming from Rupert Neve.  No tricks, just a good design, decent transformer and high-quality components.  You might end up wanting something more expensive, but it is good enough to stay w/ you for as long as you record audio.  There are other deals, but the ISA is a good complement to the clean, transformerless mic preamps found in every interface.

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