Berkeley Alpha DAC with or without preamp ?

I am happy owner of Berkeley Alpha DAC series 1+ Alpha USB . It is connected directly to active speakers Backes&Muller
In manual instructions Berkeley says that connect it directly to power amp without preamp is the best way not to loose fidelity
I think Berkeley has very good Digital volume control and its cooperate with my B&M speakers very nice
But some people says that good active preamps with Analogue volume control in chain should improvement sound quality
I am thinking to buy really good preamp
tube: Audio Research or maybe Joule Electra ( but it has balanced xlr outputs ) or maybe solid stare Mark Levinson no 26 ( it has xlr )But I think I prefer tube sound like Joule more.

the question is If using good preamp before Berkeley will be really significant improvement of sound quality ??

or maybe the better way is to change my MacMini/ Audirvana source by Digital transport like Aurender , Auralic , Moon, Innuos etc. ?

thanks for any comments and advice

I’ve had several dacs including the Berkeley reference dac. I’ve used it directly to an amp without a preamp with great results. The preamp section or volume control section of the Berkeley is superb but I always ultimately prefer a preamp.

While going directly to an amp seems to give more detail and ultimate resolution, adding a preamp, especially tube, sweetens up and rounds out the music. I think a great preamp adds musicality and gives me a much better overall experience. I can listen longer and enjoy it more.

This is my opinion.

Are you happy with your sound now? Do you think you are missing anything now?
Joeinid Thanks for Your opinion
I dont think I miss anything cause sound is nice and smooth
But I bought Joule Electra LA-150 mkII which should arrive next week
and I hope it will be good choice
The Joule should be fabulous. Please update when you get a chance. Joule has always had a very musical and sweeter than typical presentation. I’m sure you will love it.
as You said : Joule is fabulous
sweet , full, organic , rich sound with great dynamic 
thanks for Your help