The cheap price is mainly down to the 600 being a 30ish year old console. At the time, they were stunning for the price--you had to spend much, much more to get anything with nicer pre amps or better EQ. From memory, the crosstalk figure between channels was something like 25dB better than most of the competitors. There were a few issues with the first release (they quickly changed the pre amp circuitry) and they became a bit of a workhorse.
In good condition, it would still be a very good desk but I'd worry the ravages of 25 years abuse (or just aging) might have taken their toll. I remember back in the early 2000s it was not unheard of to buy one cheap and have an engineer replace literally every capacitor to get them going again!