I'm considering replacing my passive preamp with a tubes. Is it worth it?

I have used Audible Illusions L1 tube pre-amp very nice. 
Do a google galen carol (out of TX) he sometimes has refurbished AI under $1K. 

... I just recently upgraded into a new-to-me Rogue Ninety-Nine and Bryston 4B-ST, and am very happy with the results coming out of my Gershman Avant Garde RX-20 speakers...

... sorry - that is a Rogue Ninety-Nine Magnum...

One of the finest designers and manufacturers of pre amps, Mr. Emmanual Go of First Sound, started by making passives and now produces only actives.   

That is in itself is a statement given the superb quality of his products.  Vacuum tube pre amps at their zenith.  

I just auditioned the .7 with a NAD C 368 and the Rogue Sphinx -- so two Class D power amplifiers, but SS pre on the NAD and tube pre on the Rogue. On those two, it was a pretty marked difference in treble and how forward the sound felt, with noticeable brightness with the NAD. We were listening to a Wynton Marsalis CD, and it was like his mute popped halfway out of his horn on the NAD compared to the Rogue. And I'm honestly not sure which I prefer, but it's certainly different.