makum101,
it's a v.good topic. u don't need "loads" of salt but i'm pretty sure it's a key ingredient and although we have low salt i would not use it in a curry. i use 1 tsp per 800 g onion and it works well. i also cook (simmer) the base for at least 3 hrs (2 hrs lid on, blend, 1+ hrs lid off, adding back water to maintain the initial blended volume).
i'm up north so it's not the fluoride.
i had it in my mind a while back that my bases were not sweet enough. i've never had a bitter base though and have made SNS & Saffron. at the time i started adding Jaggery {palm sugar} following ifindforu's prompt. i tried several amounts and ended up using 1 off sugar lump size per 800g onion (~10mm cube). at some point i found i did not need it and have stopped using it. it did work however as a sticking plaster.
i also use anis as a whole spice which i feel has some effect on the sweetness of the onion (1 off per 800g onion taking it out before blending).
i only ever use bog std onions ie brown and only boil them. i don't cut them up small (1/4's).
i also limit the amount of initial water that i add (for the lid on stage). 20% of the onion by volume (300ml per 800g onion). i then add the extra water after blending. i feel by limiting the water in the earlier stage it helps with the sweetness. i fully believe this is not done by BIR's and therefore not a necessity. nevertheless it's a further option to consider.
i can probably find the relevant links if u need.