Limited bandwidth which SBR can effectively overcome. Most analog SW broadcasters don't equalize their broadcasts for the limited bandwidth of SW and the audio just sounds terrible, i.e. muffled and boomy, as a result. Non-equalized audio especially music on analog SW sounds awful unless broadcasted with at least 30 kHz of bandwidth. Personally, I can't listen to non-equalized WTWW on 5085 kHz for more than 5 minutes without getting a headache.
I've attached a sample recording of an analog SW transmission, Radio Marti, on 7355 kHz, after applying a steady roll-off of lower frequencies from ~ 800 Hz to 50 Hz (cutoff). The mid-range frequencies are largely flat and unaltered while the high frequencies are less steeply rolled-off beyond 6 kHz in case Cuba decides to broadcast 30 kHz, distorted analog... LoL I don't really alter this configuration much at all even though audio frequencies beyond 6 kHz are largely non-existent on SW. A multi-band parametric equalizer, qpaeq, was applied to the 7335 kHz transmission received via an SDRPlay RSP-1A and CubicSDR along with Speex NR from DReaM at -9 dB.
A properly equalized broadcast will sound progressively better with better audio equipment while retaining its balance.