Wow, that was really nice to look inside the guts of a S-550. I can attest for the quality of the build and the components used. I still have my W-30 (bought new in 1991) and it works and sounds absolutely fine, with no flimsy connections nor blown caps!
The eight track outputs were astonishing at the time, although I never used more than four in mine (when I recorded to my dad's Akai GX-270D-SS quadraphonic reel-to-reel)
On the W-30, the SCSI-1 interface option was about $300 at that time, and an external 80MB SCSI hard drive was more than $1000. The W-30 alone was $2300, therefore it was something not for the faint of heart. I can find the SCSI-1 IC on eBay (not sure if they are fake or not), but a working SCSI-1 HD in good shape (and a reasonable price) is another story.
A few remarks about the ICs you found.
The custom Roland ICs are really a nightmare to find, and can only be replaced by scavenging other gear...
At 14:23, these are standard EPROMs (16k x 8bit), and the SRM20256s are 32k x 8bit RAMs. I am not sure if the S-550 had internal wavetables in ROM, but surely the RAMs are the sample memory.
At 14:46, it is interesting to see "S-50" on the custom IC, which is probably shared with its S-50 sampler brother.
At 14:53, that is the Floppy disk controller from Western Digital.
At 15:09, the NEC is probably a display controller of some sorts.
At 15:36, among the sea of RAMs I was amazed to see a
TMS3556 from TI, a video processor for the external monitor connection.
Overall, nice video!