Passive preamp issues or?

Very familiar with the Nuforce and the Axiom Walker unit. As good as the passive unit is ( and I find it to be excellent ), the less in the signal path ( 1 less volume control, 1 less set of ics, etc. ), generally the better. The STA 200 is very easy to drive, and this is what you are hearing. As far as the hum is concerned, the Axiom unit shunts the amp's inputs to ground, therefore ime, less / no hum. 

renisnceman
The change in detail and depth was not subtle. Soundstage extended beyond the speakers- everything just sounded better. However, with the high gain of the Nuforce there’s a hum in the speakers

The Nuforce STA200 has an input sensitivity of needing just 0.6v input for full power output of 80watt. Your Jolida DSD Dac can have up to 2.3v output at full volume, so as you can see there is absolutely no need for any active preamps with even more gain, as you already have 4 x more from the Jolida than what the amp can take

But!!! I can’t find anywhere what the output impedance is of the Jolida DSD Dac, being tube it "could" be quite high or low depending on what configuration the tube is in.
Jolida state this "0 V to 2.3V (±1dB, 47K ohms)" But I can’t!! believe anyone would have the output impedance of their equipment at 47kohms!!. But if this is correct it "could" be the source of your hum problem.
Because the unwritten output impedance to input impedance "ratio law" is a min of 1:10 or higher. "IF" the 47kohm output impedance of the Jolida is correct?, you have basically have a ratio of 1:1 into the Nuforce "that’s not good" won’t hurt anything, but "could" be the source of your hum. Or there may even be an earthing problem between the two. 
Try to find output what Jolida say it’s output impedance definitely is.

Cheers George

The hum is non-existent with the volume down-  I’ve got nos Telefunkens in the dac—wondering if switching to lower gain 5751’s would make a difference?

The gain will be the same, the tube doesn't decide it, the components around it do. Do you know at what stage the volume control is done on the dac?  

Cheers George