Sounds like you're having a sub-woofer party in a closet.
Last updated 6 months ago
See the mod here - https://www.head-fi.org/threads/fostex-t50rp-incremental-mods-and-measurements.618659/post-16356751
Updated mod - https://www.head-fi.org/threads/fostex-t50rp-incremental-mods-and-measurements.618659/post-16376166