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

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.

The thing about a tube preamp is that the preamp is just the start of things. IME, you begin with which tubes to go with. I prefer the 12AX tubes. I have an LS15 preamp with modifications to the SS DC section, soft recovery diodes, better caps, as well as bypassing the caps in the audio section with Mundorf Silver Golds. Despite all the enhancements that made, it was still less than I wanted. Then I saw a little Chinese made 12AX7 based preamp at a place that sells things that Amazon either couldn’t or chose not to sell. It’s 12AX7s were broken, and some plastic covers off the rear pillars were missing, but it was perfect for me. It had a tube rectifier so I didn’t have to quiet the DC down to keep noise out of the low level details. In fact, other than rolling tubes, the only thing that I did to it was to bypass the large output coupling caps with Vcaps. Put in some Telefunkens and put a fork in it, you’re done. Not very versatile, but if you want tube flavor, it’s the way to go. My AR and Coda preamps are sitting idle. I should and likely will try some copper Vcaps in both preamps, but I think 12AX7s just have a bit more magic if you will, than 6DJ8’S do. So, 270 for the preamp 130+/- for caps, and tubes for what you can find them for, my source has Telefunkens pulled from test equipment, they test new and seem to last. YMMV

This is the preamp I now use;

https://smile.amazon.com/Nobsound-NS-01P-Preamp-Preamplifier-ALPS27/dp/B00LUCZLZQ/ref=sr_1_4?ie=UTF8...

I've actually gone from a tube pre to a passive. I used to own an Octave Audio Phonomodule. At the time, i thought it sounded good. That was until my mate brought his Dave Slagle autoformer TVC over. The difference was day and night. Soon after i built my own and have never looked back. I will never go active again. 
But as they say, it's all system dependant. 

A used  Rogue Metis preamp should be in your price range. For the price a nice tube pre with easy tube rolling. Have been using one for a year now and I'm very pleased with it.