As I understand it, part of the alleged problem with the original 680 was that the capacitors were too small when it came to providing phantom power, and Tascam fixed this in the mkii along with providing improved op-amps. It remains to be seen whether changing the mkii op amps would make a substantial difference.
With the Zoom F8 on the horizon offering 8 individually adjustable channels, nice metering, and possibly, timecode, in a SD style case for $1,000, it might be smart to pause and let things settle down before committing $600 to the mkii and then spending another $350 on modding the mkii for what might or might not be that much improvement.