I just bought the FOSI Q6 and FOSI GR70 to pair with the Cambridge Edge W power amplifier to drive the KEF Q Concerto Meta speakers. However, I felt that the tone and details were slightly lacking, so I purchased a linear power supply and a pair of 6AU6 vacuum tubes. Both arrived today. After switching to the linear power supply, the sound improved significantly, with the highs becoming smoother and more comfortable. Then, after swapping in the 6AU6 vacuum tubes, the sound quality took another leap, with both bass and treble sounding more beautiful. Tube amps really are endlessly enjoyable! 
