Like many of you, I've spent years trying loads of different base + final sauces but they were always a little lacking. Here's the secrets I think are key
For the base
1. Use a good, subtle base sauce - add 2 or 3 tbl liquid chicken stock in addition to the usual onion, garlic, ginger and tomato mix.
2. Keep spices to a minimal - tumeric, cumin, corriander, chille powder per the recipe.
3. Use a jar of Passata instead of tinned tomatoes - less bitter.
I prefer to fry the onions first, then add the water (keeps the smell down)
For the final curry
1. Add a few tbls base sauce to a few tbl oil
2. Add cooked (or raw) chiken - cook as required
3. Add 1/2 tsp cumin, 1/2 tsp corriander, 1/2 tsp garamasala (methi optional) and fresh and/or powerded chilli
4. add 2tbl Tomato ketchup <----------- this makes a HUGE difference
5. add a couple of ladels of sauce
6. 1 tsp salt
7. add a few tbl fresh corriander
Use good quality spices - I love Gatco.
In summary, keep the spices subtle (use sparingly) and add Ketcup to the base just after the dry spices. Don't add dry spices after the ladels of base sauce - this ruins it.
Good luck.......