How old are your speakers assuming you purchased preowned?
I have not heard the Synthesis gear but @larryi and @waltersalas use them and I highly respect their ears. With that said, the OTO Phono SE Signature is a pretty safe bet and can be your end game, especially coming from NAD (no offense intended). You may not require a phono stage but you might someday. In addition, the phono version has a single 12au7 tube in the circuit along with the pair of 12ax7’s and quad of el84’s. Can be tube rolled to suit your taste although it sounds lovely with cruddy stock tubes, so there’s that. It is also optimized for a 6ohm load (matching your speakers) and the transformers in the SE Signature are exceptional. Just a lovely amplifier.
A word of caution, buying preowned Audio Note requires the buyer to research the given item so that you can ensure you get the latest version as Audio Note doesn’t change model numbers, they just incorporate running changes.
You CAN get an Audio Note OTO Phono SE Signature preowned AND a Bluesound Node Icon for your stated budget. That would be a crazy good combo. The upgrade path from there would be (in order) Audio Note biwire LX speaker cables, Audio Note LX or ISIS interconnects and then an upgrade to the Node ICON streamer/dac. This would be crazy good…
I also LOVE the Leben CS300xs and it responds a little better to tube rolling. A word of caution though, the Switchcraft input jacks require great care as they are a terrific sounding albeit mildy delicate jack. Use Switchcraft termintated interconnects….

