upgrade advice

the Bluesound's DAC is fairly decent, but not close to the Hegel or the Spendors, which are both quite good

I vote for trying a different DAC. FWIW I purchased a Bluesound Node 2 back in April and retired my 2012 Mac mini (i7, 256GB SSD, 16GB RAM, Amarra Symphony, Audirvana +, JRMC, Pure Music) .. the Node 2 is really quite good. I prefer both of my DACs (Chord Qute EX and Schiit Bifrost Multibit) to the on-board DAC in the Node 2. Not surprising really. BTW, the DAC in the Node 2 is a Burr-Brown PCM5122 and isn't bad at all. 

I tried the H160 with my Bifrost and the Bifrost provided superior sound  to the onboard DAC

Granted, It is tricked out with good cables and a V-link192 to handle the USB interface.

I was very surprised by how much difference the Bifrost made, given the reputation of Hegal - it put me off their products somewhat.

A new DAC might get you to where you want to be, but be sure to get one that does not rely on USB power. Both the Bifrost and Gungnir utilize power from the USB cable to power their USB interface - not good - a Musical Fidelity DAC would be my choice, based on how good their V-Link192 performs.

Regards...

It sounds like a reasonable upgrade path is use the Node 2 as a wireless streaming transport and feed a better DAC using its optical/coax outputs. I suppose the same DAC could be used to upgrade a CD player sound as well. Makes it a little easier to justify the cost for a good unit.

a lot of this is simply personal preference, as long as the DAC receives a high quality, low jitter signal...