I've used the 3005 package with receivers from Yamaha, Denon, Onkyo, Arcam, Pioneer, Marantz, and maybe one or two more. I've found that they continue to improve using receivers up to around the £1,500 mark. There will be some benefits to receivers above that price point, but they'll be minimal.
Between an LX83 and an LX75, it'll depend what features you need/want. The 75 will have AirPlay and a rather in depth and snazzy iPod/iPad control app. Another benefit is that the LX75 now uses a DirectFET digital amplifier, instead of the ICE digital amplifier used in the LX83. I've just changed from an 83 to an 85, so it'll be a few days before I can comment on the difference between the two, but Pioneer have missed out the 74 and 84 numbers because these new receivers are supposed to be so much better than the 73 and 83. Time will tell.
The 83's digital amplification really brings speaker's to life. The amplification is lively and very detailed, with a tight grip over the speakers, including those that are 4ohms. It's more efficient than other receivers that use normal Class A/B amplification, and runs a little cooler, and that seems to go for the 75 and 85 amplification too (mines been on for a few hours now). If you like neutrality and detail, the Pioneer, 75 or 83, could well be what you're after. At £2k, the 83 would be a little OTT for the 3005's, but at the reduced prices, that's not an issue.
The Yamaha 3067 can be had for a bit cheaper, and based on pairing them with 3005's, is every bit as good as the Pioneer 83. It's warmer and softer than the Pioneer, so doesn't have that neutral, biting edge to sound effects. The Yamaha's strong point is it's soundfield, which is one of the most cohesive available, and mixed with the UniQ drivers of the KEF's, with their strong point of imaging and excellent off axis response, combines to give one of the best sound fields around.
So it's your choice - the neutral drive and grip of the precision Pioneer, or the enveloping soundfield of the smoother, warmer Yamaha. But don't be fooled into thinking the Yamaha lacks drive, or the Pioneer has a substandard soundfield....