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The 740a is a decent lower mid range amp and I don't think you will see a massive sound performance changing to a 840a using your current wharfedale speakers unless your are thinking of upgrading these speakers in the future too.
You will have more grunt on tap from the 840a for sure and may hear a slightly improved sound quality over the 740a.
It's a shame you could not hear the 840a connected to your wharfedale speakers before deciding if this was a worth while upgrade purchase over your 740a.
What is your front end source you are using with your amp , ie CD player, streaming device ?
Sounds like to me that you have got your heart set on buying the CA851A amp.
If this is the case, see if RS can either offer you a free home return demo or they can give you a in store demo connected to the most expensive speakers they have on display in the shop, to give you an idea how good the amp preforms.
Can't comment on the KEF speakers, because I don't no anything about them.
if you are seriously looking at the CA851 amp, then personally I think you need to be looking at £1500 plus for brand new speakers to do the amp justice.
Or you could save some money on the speakers and go down the second hand route somewhere between £500 to £1500.