You can do this with a dual gang linear pot before the volume pot of your amp.
The drawing shows both channels. As you wind the 250K pot one way the wiper attenuates one channel more and the other less, then back the other way vice versa (one channel will get louder as the other gets softer). There will be no centre click but there will clearly be a midway point at which the 2 channels are exactly in balance.
A 250k linear balance pot could be used with a 100k log volume pot to give an input impedance of 70k to 250k, with the mid balance position 180k. Max source impedance ( at mid balance position ) would rise to 38k from 25k for the vol pot used alone.
A 50k linear balance pot could be used with a 20k log volume pot to give an input impedance of 14k to 50k, with the mid balance position 36k. Max source impedance ( at mid balance position ) would rise to 6k6 from 5k for the vol pot used alone.
Note that in both cases, at mid balance position, the input signal will be attenuated by x0.5.
The increase in source impedance may make the 250k/100k set up unsuitable as a passive pre to drive cables but it should be OK in an integrated amp or active pre. The 50k/20k set up would still be fine as a passive pre driving short cables into a high input impedance but its 36k input impedance should be borne in mind for source components ( SS will be OK but valve sources may struggle ).
