anyone have experience with "replacement cost" insurance, and Allstate?

As a 36 year adjuster, I mimic what the others have told you.

An adjuster's biggest concern is getting audited for not having proper documentation to support what settlement he achieved. Flood the adjuster with documentation that can be verified that will substantiate the value. Provide all alternatives as concerns repair vs replace so that the best decision can be made.

What a good adjuster will come to realize is that a Mark Levinson amp is like a car. The parts not destroyed have after-market value.

If the election by the carrier is to replace, they get the salvage of the Amp. If you negotiate to keep the salvage for parts [lest you do find a used replacement], then they pay "replacement cost LESS salvage value".

Dweller2 is part correct.

Most carriers will pay "Actual Cash Value" up front. If one is forced to obtain a replacement and invoke the "Replacement Cost" provisions of the policy [regardless of whether the purchased item is new or used], then the damaged owner will need to purchase whatever they need and submit the receipts to collect back the difference between ACV and RC.