I've only been listening to the Sony Walkman NWZ-A840 series for a couple of hours and has so far I've been impressed by it. The sound quality is pretty good, but I'll give it two weeks of intense listening and comparison with other DAPs before I comment further.
The amp here isn't as powerful (or it may be a bureaucratic decision) as the one that powered the X-series or S730/S630 that much is true. With the old X-series I rarely need to go above volume 15/30, but with the A830 I would need go increase it to 20s to get a comparable loudness. Not a problem with my IE 8, but this may be an issue with people with more demanding headphones. I haven't touched the bundled headphones yet, but they look good and should sound better than most bundled headphones. I still suggest investing in an aftermarket higher end headphone.
Build quality wise, the A840 passes with flying colours. The OLED screen is dominated by a visible glass layer that gives it an extra class though I worry about the durability (plastic is generally more crash-proof than glass). The buttons on the front are nicely build but do not provide enough feedback. It should have been raised so as to provide a better tactile experience for people who want to control it 'blindly'.
Transferring music is quick! Despite being connected via MTP, it is so much quicker to move music onto the A840 compared to the X1060. More later!
Just a note on Sony's confusing way of naming products: NW means it is for Japan and NWZ is the international version. A840 is the series as a whole and the last digit represents the storage capacity. So A845 is for 16GB, A846 for 32GB (so far only available in Asia Pacific and Japan) and A847 is for 64GB (which does not exist outside of Japan).