I have Tascam DR-40, a Busman moded DR-680 which I'm selling, because I got also the fully professional HS-P82. The last is of course much more expensive and I think in terms of sound and built quality the best. But it's heavy. I think the difference in sound quality to Sound Devices is not big; they are just a bit different.
The Busman moded DR-680 comes sound-wise very close to the HS-P82, with a very slightly less headroom and very tiny bit more noise in the preamps, almost not worth mentioning. It's much lighter than the HS-P82, but it has only four XLR inputs and two balanced TRS inputs accepting microphones and line inputs, but you can expand it to eight channels of simultaneous recorsing by feeding the last two channels from any preamp that gives out a digital SPDF or AES/EBU signal (but not the Audient Mic pre, which is very good sounding, but somehow doesn't work with other SPDF devices).
The DR-40 is smaller, of course, but you can record 4 channels using two built-in mics and two phantom powered XLR mic inputs, and the recording quality is not ways behind the DR-680.
So it all depends on what you want to do. With the DR-40 one can achieve a very good quality already out of the box and especially when using external microphones. With the other two I would dare to produce commercial quality records just using the right microphones, especially with the HS-P82.
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