Passive preamp issues or?

If the amp really has a 1MegOhm input impedance

Oh dear! This is getting like a Monty Python skit.
He doesn’t have an amp with 1megohm input impedance, I’m just saying that the Rogue M120 the only amp I know of that would be an "impedance match" with a tube output source like his with 47kohm impedance.
And btw I’ve had the Rogue M120 here some some time, they don’t hum, with active or passive driving them

Cheers George

There it is! The problem is either the high output impedance of the Jolida at 47kohm (which is wrong for anything mind you) save the input impedance of Rogue M120 monoblocks, which are for memory 1megohm, or similar, they would give a good ratio of around 1:20.
https://www.stereophile.com/content/rogue-audio-magnum-m-120-monoblock-power-amplifier-measurements

Or maybe it’s the Jolida output coupling cap with that impedance, or could be earthing.

But I don’t think you’ll ever find any manufacturer with a source product that has around 47kohm output impedance.

Cheers George

As I asked you to put in your Blue Note 2 in, did you? (which also has volume) Direct into the poweramp, and then you can see if the problems disappears.
If it does then the Jolida tube output is the problem, and even if it doesn’t make noise will still be an impedance mismatch at 47kohm with the input impedance of the Axiom pre also.

Cheers George

I’d say without ever doing the experiment to see on the test bench or hear also, this 47kohm output impedance "could" be the problem. As it will also be "capacitor coupled" as well being tube which "could" make it even more of a problem if the caps not large enough, or an electrolytic, who knows, as it’s not following the output input >1:10 "rules" as we know them.

Cheers George

If it is 47kohm output impedance, that’s insanely high, never seen anything like it.

If you get no answer, also try another source with output rca’s like the Blue Note 2 streamer as it has volume control also, see if it still hums then.

Cheers George

I’m not saying a grounding issue, that’s why I asked for you to find out what the output impedance is of the dac (at all volume levels) as this sound like it’s the dac impedance match maybe problem, or it's hum in the dac just being presented.

Cheers George

Sorry I should of elaborated a little better when I said the tube doesn’t decide the gain, in that it (the tube) doesn’t decide the gain in a tube output "buffer circuit" as you most probably have in this dac.

Cheers George

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

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