Thanks for the info!
Yeah, I suspected that might be the case; I'd need to change how the setup a bit to make it work.

No, accessing a subdomain on an onion address (storage.xxxxx.onion) has no meaning in Tor, it will always go to the onion service by the xxxxx public key. The HTTP client/web browser will however send `Host: storage.xxxxx.onion` so it's useful for virtual hosting on the same webserver/onion identity. Since the main site and storage server are physically separate you should instead run two onion services, one on each server, which will have different identities (xxxxx.onion for the server on animetosho.torrentbay.st, yyyyy.onion for the server on animetosho.torrentbay.st/s__storage).

Attachments are listed in the show=torrent request.
See Attachment Files Table for info on how to construct the attachment URL.

Thanks for the suggestion. This site doesn't actually use load balancing; subdomains direct requests to the secondary server (e.g. animetosho.torrentbay.st/s__storage).
Do you know if subdomains can be set up to direct requests to different servers (without a load balancer)?

Onionbalance may be what you're looking for. (Load balancing an onion url against two servers)

Can the JSON API be used to download attachments and subtitles?