
I tested both digital IN and OUT. Here's what I've found...
1. When in RECORD mode, there IS a small bug with JJ OS2xl ver 2.21.
The audio signal was not coming in when I selected DIGITAL source. So I selected to ANALOG and right back to DIGITAL. My signal worked. I recorded a whole song into my MPC1000 and there were no problems with it. So the workaround is to toggle the source between ANALOG and DIGITAL as suggested. Also make sure you're using a real 75 ohm cable (not the standard RCA).
2. I tested INPUT THRU and it works fine with no problems.
3. I've tested DIGITAL OUT as well. There has been a minor bug that's existed for many years.
Upon powering up the MPC1000, as soon as the JJ OS (every version I've used over the past 5 years), you will notice a loud click or pop once the OS has finished booting up. It's only a quick pop. However, I would suggest having your MPC already running before powering up a DAW to avoid any damage to your speakers. I've owned the same pair of monitors for the past 10 years and haven't had any damage done. Though I should mention, it's merely just a slight annoyance at best.
ALSO, I did notice more than that click or pop upon the OS being booted up. I got 1 second of static, which went away immediately. It sounds like some 8-bit type of noise, similar to an old Atari game console.
4. My conclusion... both DIGITAL IN and OUT still work with no problems, aside of what I've mentioned.
I'm really not sure if that loud click from #3 on this list is from the JJ OS or just merely something to do with the unit itself. But honestly, I'm not about to go back and install the AKAI OS to find out, since it hasn't caused any damage or further problems in the audio signal.
If anyone else would like to run the same tests I've done, I'd like to know the results. But for now, the original problem with the DIGITAL IN should be looked further into, so we can confirm the same results to report to JJ as necessary.