the cart or the horse?

Personally I would start with the preamp and get one that has a good DAC in it to bypass the DAC in the streamer.

You could try a different streamer for cheap too  these guys seem to think the ifi zen is better  I have not heard either  

i don’t agree that efficient speakers need low power. Look at what JBL does in the pro market. They put large efficient horns with silly amounts of power. Low power amps don’t sound better they just have less power. Unless you love tubes and need to keep the cost down there is no reason for low power. 

@wavendon

Without question you’re going to receive a multitude of different suggestions, so I may as well offer mine. IMO the best part of your audio system are the Omega speakers, I’d keep them. I believe that you’ll get much more involving and emotionally engaging jazz playback with a tube amplifier.

Fortunately you don’t need high power and I disagree with the the comment above with regard to low power amplification. The Omegas allow you to to focus exclusively on the quality of the amplifier rather than getting more watts.

For example (And there’re numerous good ones)

Luxman SQ -n 150

Coincident Dynamo el 34SEP/300b SET

Dennis Had Fire bottle SEP

Decware SEP several model choices.

Anyone of these choices would sound "much" better and involving than your current amplifier/preamplifier chain. Your jazz recordings will spring to life and pull you in deeper to the music. Your Omegas are not the problem. After improving the amplification I would next upgrade to a higher quality seperate DAC connected to your current music server.

Charles

I agree with @charles1dad. The Omegas are the jewel in your system.  Tubes or perhaps SS class A would probably work great. 
 

   I used the internal DAC on the Bluesound for a while. It was adequate,but a decent external DAC will most definitely be a large improvement. 
   Enjoy the journey. 

Loudspeakers first perhaps Triangle Esprit Titus EZ. Next would be your amp, we can talk about that once you change your loudspeakers. 

The Omega 7's are the hidden jewels of the Omega line.  Go for a quality hybrid tube integrated ... Rogue Sphinx 3 (new @$1600) ... and you should be in business.

Stereophile review - Rogue Sphinx

Sphinx Info Sheet

Rich