For media, it's hard to beat UnRaid. There are lots of good options (including the FreeNAS you already have), but here are a few things UnRaid provides that are good for media.
- No need for special RAID hardware controller
- No need for matching disk drives (heterogeneous array). Just buy the "sweet spot" drive whenever you need more storage
- Can easily be upgraded. Simply add/remove/replace a drive and go.
- Parity drive allows for lossless recovery if one drive fails. This is a good balance of efficiency/safety for non-critical files like media. Only one drive worth of storage is "wasted/unusable" for your files. On mirrored RAID systems 50% of the total capacity is lost.
- Is optimized for Read access, which is what you want in a media server. Writes are slow, but that doesn't matter much if used as a media server.
- Only spins up the drive in use, saving power. Striped arrays spin up all the drives.
- Runs off of a USB thumb drive, freeing up one more drive bay for storage
Many other NAS solutions share some of these benefits as well, but these are the ones UnRaid has which make it appropriate for media storage. My personal advice would be to just stick with what you have - don't fix what ain't broke
