I had a similar experience. Rebuilt some circuitry in my Parasound JC-1 power amps, and the midrange detail was significantly enhanced. But it appeared I had lost bottom end. All my Parasound equipment is direct coupled and uses DC servo controllers (no coupling caps).
I put a wide band pink noise source on the power amps and they were dead flat to below 10 Hz. So all I can conclude is the enhanced upper end detail has shifted my perception of a thinning bass, but it is not there in reality.
I put a wide band pink noise source on the power amps and they were dead flat to below 10 Hz. So all I can conclude is the enhanced upper end detail has shifted my perception of a thinning bass, but it is not there in reality.