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

Get a old CJ tube preamp this would sound musical with a lush tube sound,Enjoy.

This is within your budget!
http://www.theabsolutesound.com/articles/brown-audio-labs-sp-1b-preamplifier/

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

That Brown Labs sounds as though it's an excellent buy.  It looks great too.  I am leaning way over to it right now.  

Try a Bryston bp 20 , 25 or sp2 . One of the sweetest volume controls around . Oughta work well with your amp .