Secret Santa, I think we've read each others posts over the last year or so, so I think I know where you're coming from.
Perhaps sourness is the wrong word, but the tomatos certainly add something, perhaps 'tang' would be a better description, certainly not sour in the way that lemon juice -for example- is sour. I'd be surprised if you have not tried it, since reading your posts it's obvious that you've tried pretty much everything, but if you havn't, please do, just once!
The Madras I make, at the risk of blowing my own trumpet, is easily better than any BIR near me. What it isn't better than however, is the cheap but amazing savory hot curries I had in Leeds between 1982-1985. I just can't get anywhere near that flavour! Reading between the lines, I suspect it's these savory curries you're trying to replicate. The ones that you can't find anywhere anymore! If you find the answer PLEASE me know. I can still remember the amazing smell of these dishes like it was yesterday!!
When I first came across dried methi leaf, I hoped that would be the answer, but it wasn't!