A preamp does these things:
1) provide volume control
2) provide any needed gain
3) provide switching for various sources
4) control artifact from interconnect cables.
Of these the last is the least understood. Since you don't need any gain my recommendation is a buffered volume control. This is a passive control that has buffering circuitry with no gain that allows the control to work properly without artifacts being generated by the interaction that typically occurs between the volume control and the interconnect cable driving the power amp.
1) provide volume control
2) provide any needed gain
3) provide switching for various sources
4) control artifact from interconnect cables.
Of these the last is the least understood. Since you don't need any gain my recommendation is a buffered volume control. This is a passive control that has buffering circuitry with no gain that allows the control to work properly without artifacts being generated by the interaction that typically occurs between the volume control and the interconnect cable driving the power amp.