I absolutely loved my Tascam DR-680mkii! I only sold it because I got strapped for cash. I then bought the Tascam DR-70D and it is about perfect (for all the reasons others have already mentioned). Plus, I could set it and leave it without worry, and when that festival dust started blowing, I knew is would be easy to replace if anything happened to it. BUT, I certainly began missing those channels 5/6 and 7/8 that the DR-680mkii provided. 2 mixed stereo pairs plus a soundboard is about perfect. I received a MixPre6 as a christmas gift, so I haven't used my DR-70 while I get to know the mixpre6. But, I can't seem to allow myself to sell the DR-70. It's the perfect 2nd recorder, or primary under sketchy conditions.
All that said, you may be able to find a used Zoom F8 for around $700.