any suggestions on what to replace Bluesound Node 2i with

If the issue is sound quality and not other user functionality then suggest trying an external USB  dac with the Bluesoimd streamer for comparison before dropping $4500 on a new streamer/dac.

@mapman +1 My bluesound streams via an excellent analysis plus cable to an R2R tubed dac with various upgrades and a good quality powercord. I would like to A/B the Bluesound with another streamer, but I suspect it would be hard to tell a difference.

But, OP, you asked for opinions about Lumin and I'll confess to not having experience, there. Whichever streamer you do get, if you get a chance to compare it with your Bluesound -- keeping the DAC the same -- I'd love to hear from you what kind of improvement you hear when every other variable (including the room) does not change.

Personally I have not heard significant differences between streamers to same DAC with CD res sources. . Never heard a bad one. Interface type used can matter. Asynchronous USB is the way to go with most modern DACs. I only use wireless internet connections to isolate the streamer from any noise that might exist on a wired connection and find that works great though some claim a good wired internet connection is better at least in some cases perhaps with hi res sources where wireless bandwidth perhaps could come into play.

You might also explore Small Green Computer & Roon solution.  You could get a SGC Sonic Transport i5, Optical Rendu or Microrendu and a Roon license for ~$3K. 

I did the upgrade last year and went from a Node 2i running into the Chord Qutest and I replaced the Node 2i with an Innuos Zen mkIII going into the same Qutest via USB. Streaming Qobuz. To those doubting you can't hear a difference in streamers, I am sorry to say there was a big difference going through the Zen. Bigger soundstage, both depth and width, dynamics had more body, less digital glare and harshness on female vocals. The Innuos just has a greater sense of realism, you can hear deeper into the music. The Innuos products are amazing streamers, not to mention their server ability to easily rip CD's to the internal library is insanely quick and easy. They just released their new 2.0 which for me will replace Roon. But there absolutely was a difference and it was far from subtle.