No, im not joking. The ink stability of the yellow was a significant downgrade, but I think they addressed that by now? I used a pro 4000 and pro 6000 for about 7 years as well, and I had to do so many manual feed adjustments with those machines because of the way they made the feed roller about half the diameter of the ones in the previous series. I have no idea what you're talking about in terms of durability, in my experience it was about even. I'm also speaking from operation in a production environment where we were making dozens of extremely large prints a day and swapping papers in and out constantly. As far as the heads frying the motherboard... in the 12 years ive been using 3 different 9400s ive never had anything like that happen. I did fry my own EEPROM chip from the maintenance cartridge slot by doing something stupid... a weird way to find out that the printer stores all of it's motherboard data as a backup on the maintenance cartridge board... but after troubleshooting that for a few days I was able to replace it with a blank chip that cost 50 cents and it reloaded everything.
not to mention that they didn't change anything about dot sizes between the heads, so sure, alignment might be a bit easier with a single head, but you're basically paying double for the same technology (PF05s are the same as PF04s and PF03s as well and can be cross flashed if you have a chip resetter).
As far as ergonomics are concerned... i guess it's just different strokes. i far prefer physical buttons and levers to touchscreens.
Anyway, properly calibrated and tuned up I find the quality of the prints out of these machines to be pretty much identical, except there's a slight reduction in gamut because of the omission of the green ink. I never found a ton of use for chroma optimizer, but sometimes it would be nice to get rid of gloss differential. If the new inks for the x600 series are indeed much more scratch resistant, then that is a substantial improvement, but otherwise I'm not sure what you mean by "substantially better quality" when we're talking about the same dot sizes and mostly identical printhead technology. Maybe they improved their internal dithering algorithms. idk. Enlighten me.
1) Yellow in LUCIA PRO2 (LP2) now uses dual pigment types instead of one, so print longevity should be better than LUCIA (LEX) EX and UltraChrome Pro. There will be no WiR tests.
2) Scratch resistance is vastly improved due to wax addition
3) CO is anti-reflective, improves scratch resistance even further, and reduces gloss differential and bronzing. It has much more impact on prints than the marginal green gamut difference that can be seen on 0,01% of photos with supersaturated emerald/jade greens (LEX G is rather blue-green as B is rather violet than blue), especially as the gamut of LP2 is now also improved
4) PF10 is much more durable than both PF5 altogether, so at the end of the day the cost of print head replacement is at least 2-3x lower. Both my iPF6350 and iPF8300 had motherboards failures caused by PF5s damage, there was a technician in my country that was known for fixing them (250$), but after repair you had to dump both print heads and put brand new ones. Maybe x400 was less prone to these issues.
5) Screening and dot placing order algorithms had evolved and are driven by more powerful signal processors, MBK is darker, overall LP2 inks are denser ("drier") so the ink consumption is lower and it dries faster
6) As far as ergonomics are concerned PRO-x600 has both physical buttons, levers and touchscreen, it prints much faster, loads paper faster in both automatic and manual way, aligns print head automatically (without using paper), the irritating pause between prints is now shorter, it measures the amount of paper left on a roll without printing those stupid bar codes, recognises the kind of paper being loaded, has light in printing area and translucent ink carts covers.
7) Grey inks are as neutral as Switzerland
Attached - Pro Luster at Highest Quality, with CO and without CO:
