Hi Moonie,
I've just bought the BDP-lx71 (along with PDP-5090 and SC-lx71), and my father-in-law the 51fd (played via Pioneer 436 panel). My brother-in-law has the Pana 35 (same as the 55, but without the analogue outputs and perhaps less impressive sound). They all produce superb results, but I think the 51fd would be by far your best option. It's basically the same as the lx-71, with just a lack of gold plating on its inputs and a slightly less good DAC. I think that's outweighed by the superb picture you will get from the 51fd, and the fact that your 428 and the 51fd will work extremely well together. Your 428 will accept a 1080p/24fps input from the 51fd, and display it at 72hz, plus your 51 has a preset setup specifically to enhance its playback on Pioneer plasmas. I've got no experience of the Sony 550, so couldn't comment on that. Good luck with whatever you choose.
Cheers