Metal plate beneath Linothorakes or Spolades

[PMBardunias]

Seems odd that they would start making armour of solid bronze (bell)...Then start making it in bronze but covering it in either leather, or linen, thus reducing it's prestige value (made out of bronze = more flashy) ...And then start making it out of solid bronze again (muscled)

Bronze plate never dissapeared and reapeared, this would have been a second option for armor. As for prestige value, high quality linen armor could be very expensive- a suitable gift for Kings and Goddesses. Such textile armors were probably encountered in Asia and would thus carry cache value as imports. Then there is simple fashion. Early modern cavalry shed their half-armor largely for fashion- due to a Polish musket ball that made wearing armor problematic for Gustavus Adolphus- as well as cost. Rather than helmets, they began wearing civilian hats, then put metal scull-caps or "secrets" beneatht hem to achieve some level of protection.

Also, this find just happened to be bronze, but I have a pet theory that if such under-armor plates existed they were usually iron, which does not survive as well. Bronze was expensive, while Iron was cheap in some areas of Greece- Laconian iron mines were famous as was their "iron money". Where is all the iron armor? The short answer could be it makes up much of the scales we see, iron scales being common in the near east. But perhaps due to an inability to make broad uniform plates cheaply, they made a composite of smaller plates.

Also wouldn't the linen or leather be better under the armour as a last line of defence since metal will stop cutting/piercing blows better than either linen or leather would.

Yes, but this was no impediment to the medieval coat of plates. There may be structual reasons for putting such large riveted plates on the inside. See a medieval example that is made up of more plates than I'd imagine in a T-Y below.

Seems to me that they start to make T&Y cuirass' because they wanted to equip more hoplites, for cheaper, doing that in linen or leather will do this, but not if you have a fully articulated bronze chest piece under it.

A chest piece would be far cheaper than a full plate armor. In fact "cheap" troops from Luristan in the east to Italy and Spain in the West wore a Kardiophylax or simply disk or rectangular plate on the chest. If this plate existed, it may not have covered much more than the upper front chest, or whole front panel of a T-Y. Scales being used elsewhere, or nothing at all. In the Hellenistic period we see hemithorakes, which were probably simply the front half of bronze cuirasses.