[Carl Kolchak]

Are you still on friendly, or at least professional terms with the engineer concerned?

He's right in that if you don't have the same plugins he has, the mix will not sound the same, but you could rent / buy / in some cases even demo those plugins which would restore the data.

The critical thing is the outboard equipment he has used. Even if you did have access to the exact same equipment, it's extremely unlikely you would be able to set it up exactly the same - there are just too many intricate variables to make it absolutely exact.

The thing he seems not to have mentioned is that he could (for a fee I would expect) print each track through its individual F/X chain to another track.

This would then allow you to mix / pan etc however you liked - though again, as it's all about context you may find you effectively end up starting from scratch regardless.

This also brings up the interesting debate (certainly in terms of who gets credited - or even accepts credit - for the mix let alone gets paid for it) as to his "intellectual property" involved in the mix. If he's done most of the work and considers the session to contain his hard earned "trade secrets" that you will simply be tweaking, he may not be prepared to hand that part of the data over unless it was clearly stipulated in the original agreement.

Things to think about.