Yeah. The 501 and 701 truly were oozing turds, so to speak. The F1 was little better, but it sure tried hard.
But did you ever get to listen to a Model 700? They were _sweet_. I really do miss that machine... They really shone at low levels (reverb tails and the like), since they don't really "lose resolution" (exhibit excessive quantization noise) when recording and reproducing low level signals. There was no graininess in the tails *at all*. Delta-mod also does not exhibit the same hard clipping behavior as a linear PCM system does. Sure, they will clip if you run the levels too high, but it's not a hard flat-topped slam. It's more of a gradual slew-rate limiting thing with a reduction in swing per step: this gives a remarkably very analog sound when you run out of swing. Very soft, and wonderfully familiar to the analog guy. That box was _asskicker_ in the early '80s, to be sure.
Its a great pity that the high-rate delta-mod encoding concept didn't catch on for pro audio, frankly. Nobody who started in this field after maybe 1985 will ever agree with me, but I honestly believe that it is superior to linear PCM in every respect, *when done right*. Doing DSP and editing on a delta-mod bitstream is a little more complex than on a PCM stream, but it's just a matter of one extra transform in and out, or a different choice of algorithms- you'd have to avoid baseline shift...
Sigh. We once had the perfect encoding scheme, but it died in the marketplace. It's a great pity. Too bad dbx couldn't make it work, commercially. I'd give my left nut for a 16- or 24-track Model 700-like delta-mod machine that just dumped the data to hard disk instead of tape. Too bad that disk was too big, too noisy, and too expensive to incorporate in a viable studio product, back in '83...
Time to start looking on EBay. Seriously! I want one of these things back, and maybe nobody remembers how good they were. Video-speed disk storage is dirt cheap, nowadays. Hmm.