A few questions; what speakers do you have with the Pioneer? Is there anything about the sound you feel needs to be improved or is the balance (warm vs forward/bright) not to your liking?
TBH now is a bit of a tricky time to buy an AVR.
Your money can fall two ways. Particularly as your budget isn't vastly different to your AVR's original RRP, normally in this situation I'd be seriously looking at a last year's model that began around the £800 mark and is being discounted to around £500. That way you might actually get an "upgrade" in SQ compared to your current AVR.
But much depends on how you stand with 4K. Only a small number of manufacturers have released true 4K, HDCP 2.2, HDMI 2.0 receivers, so if you were looking at a new non-discounted receiver, then your choice is pretty small at the moment anyway.
If it was my money (and you're not gunna like this), I'd wait and save a little more. I'd wait until all the new true 4K compliant receivers are on the market, then I'd wait another 9-12 months after that when they've been discounted by 30-40%. Hopefully this extra time would help you save a little more towards your budget and make the change a true upgrade. I fear if you swap now all you'll be doing is swapping model numbers for very little difference in SQ.
It's advice like this that will continue to prevent me ever getting a job at What Hifi!!!