Like many headphones, there's a hump in the impedance vs. frequency around the mid-bass.

Higher output impedance means worse damping factor, so maybe worse control over the drivers. The difference in bass quantity is easily explained by the headphone impedance graph, since the voltage the headphones see is split between the source impedance and headphone impedance.

V_L = V_s * Z_L / (Z_s + Z_L)

At a given frequency, load voltage is the source voltage multiplied by the ratio of load (headphone) impedance to the sum of the source and load impedances. If Z_s is mostly constant across frequency, we see that at a particular frequency, Z_L / (Z_s + Z_L) is larger if Z_L is larger. Thus V_L is relatively high when Z_L is high, which is around the 90 Hz region and also around up to 20 kHz. If Z_s is small, like with the headphone out, Z_L / (Z_s + Z_L) is close to 1 for all frequencies so there's little frequency shift. If Z_s is significant compared to Z_L, then you'll get significant shifts, giving you more (mid)bass and highest treble, compared to other frequencies.

Worse control over drivers--or just excessive midbass--can easily make the sound more muddy. However, the line out furthermore isn't intended to driving headphones, so you can probably also expect significantly increased distortion from that as well, on top of any issues related to worse damping factor and frequency response shift.