The power supply has at least as much influence on the sound as does the input buffer, so "all that is left" is not just the Purifi module.
Further, the Hypex input buffer was never intended as a finished product. It is about as simple a circuit as can be used to provide impedance matching and gain. While the op amps used (LM4562) are decent, they are designed to be operated at a higher voltage than is used on the Hypex buffer to get optimal performance. The 7812/7912 voltage regulators used on this board are decades old and far from state-of-the-art when it comes to noise and response speed, so there is a lot of room for improvement. And there is no frequency limiting and EMI/RFI filtering circuits, so there is lots of opportunity for environmental electrical noise to cause problems.
And, of course, there are improvements that can be had from wiring, connectors, layout, etc, but these will likely be more subtle compared to the power supply and input buffer design.
Bottom line, this amp as currently built is about the worst case scenario for the Purifi module. You can only go up from here.
Further, the Hypex input buffer was never intended as a finished product. It is about as simple a circuit as can be used to provide impedance matching and gain. While the op amps used (LM4562) are decent, they are designed to be operated at a higher voltage than is used on the Hypex buffer to get optimal performance. The 7812/7912 voltage regulators used on this board are decades old and far from state-of-the-art when it comes to noise and response speed, so there is a lot of room for improvement. And there is no frequency limiting and EMI/RFI filtering circuits, so there is lots of opportunity for environmental electrical noise to cause problems.
And, of course, there are improvements that can be had from wiring, connectors, layout, etc, but these will likely be more subtle compared to the power supply and input buffer design.
Bottom line, this amp as currently built is about the worst case scenario for the Purifi module. You can only go up from here.