I've always thought of BIR curries as if they have 3 components:
1. The main taste - the spice mix in the final cooking
2. The background taste - the depth
3. The smell - it brings everything together
It's a bit like a painting where you have the canvas, the background, and the foreground. No one element is enough and it needs the others (does that sound pretentious or what?

)
Getting the curry taste at the front end isn't
that much of a problem, but Bruce Edwards' spice mix is pretty good. The same recipe sheet (and of course this site) has helped enormously with the background flavour - there's so much depth in this one compared to the Khris Dillon 'onions-only' base. And I've also got very close to getting the smell right using the base - I never clicked on before, but the base has an important part to play in that. The Khris Dillon one didn't smell very nice when you were cooking it (and did anyone notice how the ginger and garlic turns the mixture a blue-green colour if they ever tried that one?) It was also pretty bland when you used it and just had a faint savoury smell and taste, but little else.
The Ken Edwards base has a lovely smell whilst cooking on its own, and when you heat it again in the final curry cooking along with nicely browned garlic/ginger it adds a full, sweet smell that is so, so close to the real thing. VERY close

The base also assists in the up-front flavour, too, so everything is closely woven together for the final curry.
I've got to try some of the other bases on this site - but to be fair (and in a nice way) they're mainly variations on a similar theme, and all of them are a long way away from the Khris Dillon one.
Funnily enough, a friend of mine at work used to work in a fast-food shop which also did curries on the side, and he said they just boil up onions with a mixed muslin bag of spices (but he didn't know which ones).