I'd not go this way. The resistors are critical parts in the device. The voltage and current on/through them are very small and so their quality should be very high to prevent noise (VRs due to their complexity are less quality in general). More than that - any additional contact adds noise to the very sensitive inputs (the VRs have min one floating contact inside). The size of the VRs is bigger too (together with longer wires to the VR's lugs) which raises questions about electromagnetic interference and the resulting hum on the output.
Another question is the useful range of values you can get from the linear VR. For example, to change resistance to 1k you should rotate the knob to 2.8 grad. It blocks almost effectively the sub-1k range of values.
If you are going this way in any case you can minimise some of the above problems by connecting external (not soldered directly to PCB) RCAs to (mounted as close as possible) VRs lugs and mount the two pair of parts directly on a case.Then connect by short twisted wires to the board.
Another option is to build the Piccolo, which already have a way of selection loading and gain, can be powered from the same power supply as Bugle and adds capability to use MC cartridges.
