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

Almost 100%. from what I’ve seen none have left Class A and gone into A/B, I think a manufacturer would be mad to do that with a preamp, do you know one that does?

Cheers George

Class A preamp

All active preamps are Class A, I don't know of one that's Class A/B or B.

Cheers George

Music First Audio Baby Reference. It was truly excellent, neutral, transparent, detailed, but. I felt I was missing some dynamics 

Being a passive and seeing what that source signal had to go through, I can understand why it was missing dynamics. That amount of wire and even more in the more in the transformers, would be like putting the dynamics through a funnel.

http://upload.review33.com/images/201109/201109171318487268.jpg

Cheers George

without the clicking volume control.

It’s not a "clicking" feel, it’s a smooth rotation, but it does activate different relays inside to give 128 levels via a micro processor. Relays are far better than any normal volume control, as they have far better contact pressure than a resistive track with a very light wiper contact that normal volume controls have.

" Freya uses a sophisticated microprocessor-controlled relay-stepped attenuator for perfect channel matching and zero distortion—and with 128 steps for ultra-precise level control "

Cheers George

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

Schiit Freya at $699 with trial period, it has 3 different modes, low gain tube, low gain discrete solid state, and unity gain passive, balanced or single ended and with remote control.
And with the Bryston 4b-s input sensitivity being only 1.4v input for full output, you don’t want any higher gain or the volume control will be down very low for loud listening, and you’ll have very little range on the volume control..

Can’t go wrong with this choice best of all worlds, specially seeing it’s designed by Mike Moffat who was head designer at Theta.
http://schiit.com/products/freya

Cheers George