Lol, holy crap man, you've gotta slow down! At the rate you're going you'll be going through all the new CDPs released this year and last by the end of the year!
I took a look at the tech spec PDFs that Cambridge has on the 740C and 840C and both look extremely similar. The 840C has better specs in various aspects of course, including THD, crosstalk, and digital output sampling frequencies. Don't forget either that the 840C also includes analog balanced XLR output and likely has been partially designed for that.
The WM8740 in the 740C is a strong DAC chip indeed, it's been used in a lot of CDPs (not just CA but Arcam and many other brands), but that doesn't necessarily mean it's inferior to the AD1955 in the 840C - it's all in the implementation.
Personally I wouldn't buy either CDP if I was theoretically in a situation where I'd be looking to buy. The 740C's SRP is too high for a machine with those specs (and doesn't look that much better than the 640C V2 on paper, DAC features aside), and the 840C at $375 more for a mere DAC change and XLR output is not worth it IMO. And no offense, but 1 good review from a major publication shouldn't influence your opinion.
If you're looking to try a different CDP there are many others I'd have to recommend even without hearing them, as they have a lot more interesting specs & price tags and potentially more interesting and unique sounds. If you do some research you'll find a number of other brands who released new or updated CDPs last year (and this year) that have since flown under the radar due to lack of reviews - NAD M5, Creek Destiny and Evolution, CEC CD5300 and TL-51XR, Cayin CDT-23, Raysonic CD128. Lots more to find, just have to look around.
Keep up what you're doing though, I like that you've been trying out new CDPs and posting your impressions here for us, it's extremely helpful for us folk who don't have so much money.
If only there were more people like you, then we'd have impressions on all those CDPs and more!