Hello Jon,

late comment, but I have a huge lag in listening.

One more explanation of the muffled sound when playing back at quarter speed, is the inherent low path filtering of the process of getting the sound on the reel. Assume, the original track contains tones in the 10 kHz range, these become 40kHz tones in the sped up version. When playing the quick version to record them on tape, the player has to correctly reproduce those high pitched tones and the recorder has to be able to bring those to tape. Depending on the frequency response of this chain, I expect this to be the bottle neck. When playing at quarter speed, the highest pitch you will get is only a fourth of the highest frequency the recorder could handle.

Regards,
Michael