If you have a “seriously high end system” and you are still using a Node 2i, you should rethink that part of your chain.
Oz
You are absolutely 1000% correct. The point of the Node 2i was to get my feet wet without spending a fortune. And I like the Bluesound ecosystem - I've got another Node 2i is in place for a smaller system, a speaker on a TV, etc. Bluesound is a big step up from Sonos IMO. Anyway, I'm upgrading that as we speak. Leaning strongly towards Lumin U1. My challenge is that I don't want a streamer with a DAC. The DAC is a waste of $$$ for me. The old Node 2i will end up on my home theater system where it will be fine.
But back to the OPs problem. I'm not really familiar with the gear but it strikes me as a serious but "boutique-y" with some not-so-cheap bells and whistles (cables power conditioner etc). And all that for headphones. And yet being fed by dongled iStuff! If we'd known that I think the thread would probably have taken a different (and more helpful) direction.
Anyway, here we are. OP wants a cheap streaming solution. This strongly leads me back to the Node 2i. Why? It's relatively cheap, it works well, it's very popular (i.e., not obscure) and hence easy to resell. Try it and see if it doesn't crush the iStuff. Dollars to doughnuts it will. It's also worth noting that most streamers include a DAC. This creates an opportunity to compare DACs. Who knows, maybe the Node 2i DAC will sound better than the Pegasus? Or each will shine on different material? Who knows.