Hi there Jonboi
Your setup has a major flaw, your Wharfedale diamond 9.1 can go down to 50 Hz but your tannoy fx 5.1 can only go down to 140 HZ, this means most of the extra quailty mid range you get by using the Wharfedale go down to the sub instead. This is because the onkyo 505 does not allow individual crossover setting for each speaker.
The only time you get the ture potential of your front speakers will be in direct stereo music mode when the subwoofer is not used.
That is not to say that the Wharfedales don't have any benifit, they are big improvement over the tannoy, just keep that in mind when thinking of any changes to the system in the long term and the modes you uses on the amp to get the best quailty.
Amazingly all the speakers have the same power, impedance and sensitivity so don't have any problems with load problems with the amp.
So with that in mind the best speaker cable depends on what you want and how much you have to spend. The minimum should be Oxygen free copper with a minimum of 18 preferably 16 gauge thickness. This will bring out better bass response and detail in mid range.
The more expensive silver cable are better for high frequency and may be a better choice for this setup. Personally the difference in minimal unless you spend thousand of pounds on a system. Even then its little.
I got my speaker cable from ebay, a 100m pack will cost about £20-50 depending on length and thickness.
Ebay link OFC copper
Try not to go with the thin speaker cables, that this will make the sound tin-ny at high volumes.
What's the sort of length and budget you are looking for, we will be able to help you source the right cable better.
Gurpswoo1