We bought separate insurance for our S when we bought him a more expensive cello when he was an undergrad. We had insured his previous cello on our homeowners insurance but they were unwilling to cover the cello once it was being used as part of his livelihood when he played gigs.
I don’t remember the specifics of all the companies that I investigated at the time but we ended up going with Total Dollar Insurance. There were little details that made that the best choice for him. You need to ask about the specific coverage in various situations because they are all a little bit different. For example, one of the companies would not cover an instrument if something happened to it while it was stored in a locker at the music building. Since my S routinely stored his cello in his locker at school we needed coverage for that. Also, Total Dollar will cover even if it is stolen out of an unlocked car, not all will. There were variations in other things like damage that may occur due to cold weather and during airline travel.
There are some past threads discussing this, here is one:
http://talk.collegeconfidential.com/music-major/1628411-where-do-you-purchase-instrument-insurance-is-there-one-catering-to-music-major-students-p1.html