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