Apparently didn't get a notification about new posts...
C. With some microphones, the DR680 is reported to have unacceptable amounts of preamp noise (+12dB) when the phantom power is set to ON, there is even a mod indicating how to get rid of this problem - apparently the filtering capacitor of 0.5 uf (2x1uF in series) used to filter the phantom power just at the node before the feeding resistors of 2x3.3 kOhms is much too low to eliminate the audio noise
generated by the internal 48 V converter, so replacing those capacitors is needed for a 12 dB preamp noise improvement with certain mics.
Can't say I've ever noticed a problem. When first turning phantom on there's sometimes some noise for a few seconds, but I've never had any sustained noise past that.
F. Badly designed speaker - hissy, impossible to turn off except with a jack plugged in the auto-sensing Headphone-out jack
You can just turn the volume down.
G. Badly designed headphone amp - very noisy, digital volume control unresponsive, lags behind and sounds like a zipper file, without zero-crossing detection
It could be better, but it works.
H. TRS Jacks 5/6 can cause popping on recording (one unit here had such a problem, was fixed with a contact cleaner called Progold)
I did have an issue once where there was a problem with contact and phantom and/or the mic signal was getting interrupted. Since then, I've made sure to move the plugs in and out and around a bit before recording to prevent any such issues.
K. Support with TASCAM is pretty bad, they apparently want to have 3 tries at repairing the unit before replacing, which could mean up to two months for somebody in the states, but a lot more for someone like me, living in Eastern Europe.
Some people have certainly had issues, but (fingers crossed), I've been using mine a few months short of 2 years, and haven't run into a problem yet. AC power, a few different brands of SD cards (none listed in the approved media list AFAIK), combination of line-in and mic (both dynamic and electret). The biggest issue I've had is running digital in on 7&8 can be confusing. It works fine, it's just easy to do something wrong.