I got my hands on one of these to play around with and maybe use as a backup. I did a few quick noise comparisons versus my Oade Concert Mod Marantz PMD-661. Not sure how scientific this is but here's what I did to find the "real world" noise floor of each deck. I recorded four, 10-second clips from each deck with nothing connected to the input:
1) Line with input at 0
2) Line with input at max
3) Mic w/ Phantom with input at 0
4) Mic w/ Phantom with input at max
I then opened up the clips and analyzed them for RMS signal. Results are rounded to the nearest dB:
661/DR-40
1) -73/-84
2) -73/-80
3) -73/-84
4) -53/-48
From this it looks like the Tascam takes the modded 661 for everything except Mic w/ Phantom at max gain
Listening back to the clips via headphones confirmed the numbers. In fact the background hiss is just as bad as the 661 at full gain, just a different type of "sizzle". I would say that this deck certainly for Line-in purposes and even Mic w/ Phantom (for louder sources anyway) can't be beat for the price. Not sure how the ADC compares though...
-james