I think there are three factors that help a listener understand lyrics better, or more specifically, how a singer intends to convey those lyrics.
As others have mentioned, transparency or clarity of the sound is one of those factors.
Another factor that I hear is the ability of an amplifier to convey tonal colors. For lack of a better description, some amplifiers sound more "gray" or flat. For me, the lack of tonal differences or gradations takes away an aspect of what parts of song lyrics an artist might trying to emphasize or the meaning they are trying to overlay on the lyrics.
The last factor to which I seem particularly sensitive is the ability of an amplifier to reproduce timing accurately. Specifically, some amplifiers sound slow and lead-footed to me. When the timing of an amplifier's sound output sounds right, music sounds more lyrical and flowing to me. The same lyrics that sound engaging on one amplifier can sound like they are lacking in life on another amplifier. One example in which this might be more audible are duets in which one singer's lines react to and dovetail with what their partner is singing. Amplifiers that sound slow can fail to convey the nuance of how the two singers intended their individual lyrics to fit together. My impression is that I've heard more than a few higher powered amplifiers that sound like they lack timing and sound a bit flat-footed. My layperson's guess is that complicated circuitry or electrical components that don't react fast enough might affect timing to an extent that is sonically audible. I feel that the ability of an amplifier to convey the musicality of lyrics has a lot to do with its ability to reproduce timing.
As others have mentioned, transparency or clarity of the sound is one of those factors.
Another factor that I hear is the ability of an amplifier to convey tonal colors. For lack of a better description, some amplifiers sound more "gray" or flat. For me, the lack of tonal differences or gradations takes away an aspect of what parts of song lyrics an artist might trying to emphasize or the meaning they are trying to overlay on the lyrics.
The last factor to which I seem particularly sensitive is the ability of an amplifier to reproduce timing accurately. Specifically, some amplifiers sound slow and lead-footed to me. When the timing of an amplifier's sound output sounds right, music sounds more lyrical and flowing to me. The same lyrics that sound engaging on one amplifier can sound like they are lacking in life on another amplifier. One example in which this might be more audible are duets in which one singer's lines react to and dovetail with what their partner is singing. Amplifiers that sound slow can fail to convey the nuance of how the two singers intended their individual lyrics to fit together. My impression is that I've heard more than a few higher powered amplifiers that sound like they lack timing and sound a bit flat-footed. My layperson's guess is that complicated circuitry or electrical components that don't react fast enough might affect timing to an extent that is sonically audible. I feel that the ability of an amplifier to convey the musicality of lyrics has a lot to do with its ability to reproduce timing.