Btw, how is the Bose streamer connected to the Halo, optical?
The Musette is using is using the older 16-bit Transient DAC modules, it is still pretty good and "analog" sounding to my ears but unlike the Amethyst it doesn't have the technology to specifically target the issues that MyDACII is trying to solve. One potential issue with your current setup and Metrum DACs is that it will be difficult (or rather expensive as you'd need a new device just for that) to externally over-sample the signal if you find you don't like NOS sound with 44.1kHz material (Spotify and CDs). It is a much easier task when you use a computer as a source and say using JRiver to play your content.
There are good DACs that can be used as pre-amps (volume control), and they are priced below $1,200, such as Emotiva Stealth DC-1, Teac UD-503, or maybe a used Benchmark DAC1 Pre, but I don't know if there will be a significant improvement over what's in the Halo already for your purposes.
You may want to also look into Schiit multibit DACs, they give you 15 days to audition their units but do charge a restocking fee should you decide to return it. I'd skip their 16-bit multibit offerings (the Modi and Bifrost) which brings us to Gungnir-multibit at $1,249 and 5% restocking fee + shipping on a return. It's using precise 18-bit DAC chips so you should be able to get true 16-bit resolution without issues. No volume control however.