The DACs should be last in your priority of upgrades: 1. speakers, 2. amp, 3. room, 4. DAC.
That said, try Chord DACs or other non-delta sigma DACs. Most DACs (delta-sigma) sound the same and are really common, so it's harder to distinguish unless you are just looking for a lower noise floor. The R2R dacs sound more unique and more natural for me, but usually more expensive.