I use All State. I’ve got a musical instrument policy that carries up to 20k for guitars only at market value. The amps are covered under the general house items value (amps don’t count as musical instruments, read the fine print)
I then scheduled each of my really expensive guitars separately as non professional riders. It required professional appraisals with serial numbers in the appraisal quotes and I submitted that to my insurance company. The appraisals are required for scheduled coverage.
So in summary my expensive Les Paul customs are covered as scheduled items so that the appraised value is what is given if something happens, the 20k musical instrument policy covers all of my general guitars at market value, and the amps are all covered under general house value which includes water damage.