Right on CK.
Tinned tomatoes usually have additional sugar (natural or added). Where the bitterness comes from in my opinion, is the seeds. Just try chewing one. I use both types, but Iremove the skins from fresh and the seeds from tinned. If you use fresh you should use mature soft ones that are just about to be ditched. This is what most cooks use in their curries. When they reach this state the sugar/acid balance is better and the toms are sweeter.
A little extra sugar helps to remove acidity (as the Indians do), because I am a diabetic I use Splenda powdered sweetner and it works a treat, and it doesn't cause the sauce to catch or burn. I know its not Bir but I don't think that it matters.
CP