There are quite a few synonyms that can be used to replace the phrase "pall on." To start, you might consider other words that can be used to indicate that something feels oppressive or dreary, such as "weigh down," "dampen," or "cast a cloud over." Alternatively, if you're looking for more specific synonyms that indicate a depressed or melancholic feeling, you might consider "sadden," "dishearten," or "demoralize." Other options might include "tire," "bore," or "irritate," if you're looking for words that suggest something is simply uninteresting or tedious. Ultimately, the synonym that works best will depend on the context and the specific feeling that you're trying to convey.