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

... 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.

If you can be patient and are willing to consider vintage, you may want to consider a Harman Kardon Citation I - perhaps the best tube amp of all time.  I purchased one that was reconditioned for about $1,000.  It has one of the finest phono stages too.  My only caveat is that if you have very sensitive speakers (95db+), the HK can be a little noisy (hiss) at low volumes.  

I currently am using an Audio Research LS27 tube preamp.  After close to 50 years as an audiophile, I have found that a true quality tube preamplifer is the single most important part of a high end audio system itself.  I have previously had an Ayre K1xe and presently am usng a Rega Orisis  integrated, but neither can come close to my LS27.  It seems that what goes into my Audio Research LS27, what come out sounds even better.  It's that simple.   Just one person's opinion.