Recommendations for Backup and Archive

Due to poor planning on the front end, my company is running into some storage problems, and I was wondering if I might be able to solicit some advice.

The first problem is that we have no good backup solution for our video assets. We have a 17TB XSAN that is almost full. A typical IT backup solution for a server at our office is to backup to tape (such as DLT or LTO) and then rotating the tapes past a certain age to an offsite location. This provides data security in the event of server malfunction or geographic disaster. We were doing this with our xsan before it started getting full, nightly incremental backups, and full backups every weekend. The full backups started taking longer and longer until we couldn't do them regularly (the last full backup took 2 weeks!). Our IT department has never really had to deal with this volume of data in such large chunks (where a single file could be as much as 60-100GB). Our temporary solution has been to buy 4 drobo pros. 2 are used to backup the XSAN (which is not fast, but it is faster than 2 weeks), then they are sent to the off site backup location and the other 2 begin doing another backup.
I need to know what the right/better way is. I know that XSAN creates data redundancy, but we really need to be prepared for anything. And without regular backups, we have no defense against accidental file deletion.

My second problem is that we are outgrowing our XSAN, and though we are going to expand it next month, it's not a permanent solution. I would really like to be able to use FCServer's archive functionality with a data tape library system because of it's long lasting durability. I'm sure somebody makes a solution for this, but I don't know where to look. Can someone point me in the right direction? What does a solution like that usually cost?

Thanks.

-Drew