Pi2 + HiFiBerry Digi+ Spitfire - What is my next step to improve sound quality?

Hi, My current system is a Mac Mimi running Roon core, connected via Ethernet to Raspberry Pi2 + HiFiBerry Digi+ (Roon endpoint) with Kit LPS feeding a Firestone Audio Spitfire MkII DAC with Firestone Audio LPS. It sounds good. However I would like to explore options for improved SQ? What products would you good people suggest as a next step up for someone like me with a modest budget. Cheers!