In my own system I would rank improvements from cable upgrades in the following order
1) power cords
2) speaker cables
3) interconnects
A couple of power cords to add to your list of possibilities are
1) Zcable Gold Lightning - $200
2) Cryo treated Acme - $??, but less than $100
The Zcable has a trial policy and treated Acme's do not. I have heard neither of these suggestions, but am making an educated guess based on having heard the original Lightning and untreated Acme's. I really liked both of these, but they did not increase the resolution of my system in the way some more expensive power cords were able to do.
As for speaker cables at $600, you might get better results using a product called the Zero Autoformer that allows a user to get a better impedance match between amp and loudspeaker. I don't actually know what this means technically, but in practical terms they can potentially provide really signifigant performance gains.
Zero Autoformer WebsiteI just noticed a previous Audiogon thread on this same subject and there is a poster there that owns a system very similar to my own and he essentially recommends the opposite to my ranking. One thing to note, he has a world class power conditioner and I use none. This could explain alot.
Previous Audiogon Thread