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

The two preamps listed on Audiogon are highly rated for sonics and build quality.
But the Schiit is getting raves for its price and performance. Plus, you could run it balanced to your Bryston.

An active tube preamp would add timbre and texture to the sonics of your system, since each brand of tube offers a sonic signature. It may give you more flexibility on fine tuning the volume control since it has an active gain stage.

It is most important that the output impedance of the preamp be a good match for the amp. The amp’s input impedance needs to be 10-20X greater than the preamp’s output. That would allow for transients in the lowest and highest frequencies.

t is most important that the output impedance of the preamp be a good match for the amp. 

Tested Stereophile: "Bryston measured just over 33k ohms at the balanced inputs, just over 47k ohms at the unbalanced."
This is a match for any preamp, passive or active so long as they have little or no gain as the Bryston 4b only needs 1.4v for full output, and most sources have over 2v already.

Cheers George

 @amritash what do you feel is missing from the sound coming out of your setup?

Here ya go!
http://www.quicksilveraudio.com/products/line.html