I have to use a pre amp as the built in pre amps on my audio device are very poor so i understand your situation.  I use an ART tube amp and to date hasn't caused any issue with sibilants.  But...and I could be way off the mark here.    I was having trouble too MUCH sibilants and no de-esser could help.  I spoke to an engineer at the Taxi road rally and he said it could be the mic.
I was using an inexpensive T Bone wide diaphragm and bought myself a Shure SM57 instead.  
I haven't looked back.   Could it be the frequency response of your mic not behaving?   Could you even borrow another mic to test?   Just a thought guys.  I'm by no means an expert on this. 

Generally cheap tube gear is just a gimick. Everyone is lusting after "that vintage vibe" so the manufactures jump in with gear that claims to do the trick. 
As said your probably not gaining anything above what your audio interfacde already supplies. 
Pre amps generally output to the line inputs of an interface. 
If your interface does not have line inputs then it probably has combo XLR/1/4" inputs.