Yes, you can appeal to Kuluttajariitalautakunta provided that you were travelling as a consumer (i.e. not on business). You can use your local European Consumer Centre to help you with the appeal, as it will have to be in Finnish or Swedish.
You are also correct in your assumption that they owe you € 600. Delays due to mechanical problems do not constitute exceptional circumstances, new aircraft or not. Finnair, too, know this, but it is much cheaper for them to put off payment of the standard compensation than to pay outright.