Tim, this may depend on local safety laws/rules. It is possible to have an instrument in metal case with the case not connected to protective earth (in Europe). This depends on transformer's class of isolation, i.e. on the tests and certifications that transformer must fulfill. Why P-A speaks about line filter? That's because this has a middle point connected to the metal case. As filter has capacitors between phase and neutral, phase and chassis and neutral and chassis, the chassis must be connected to PE for safety reason.
But as I stated, this is a little controversial topic, as countries have different mains distribution networks with different kinds of voltage/touch protection and different safety rules.
I was aware that it is in theory possible to have a safe mains powered product with an unearthed metal chassis - my CD player is an example of this. However, it was mentioned to me that the requirements for doing this legally are very stringent, and it is easier and safer for the DIYer to just earth the chassis. Any ground loops can be defeated by the use of a ground loop breaker circuit, as described here:
I hope that helps,
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