FZ, man, $80 for an M-30 board is the BIG SCORE, & you'da'man!
Okay. Next question, and hopefully a simple answer:
The M-30's submix is the central traffic flow section of the board, IMO, because it's needed for proper isolation of CUE from INPUTS and BUSS. It has each of 8 submix channels switchable between PRE-fader, POST-fader, and TAPE. Stay with me.
When recording your first primary trax, [and for subsequent overdubs], the SUBMIX is the key section for CUE function, in that you may listen to each individual input as mixed to an 8x2 format.
In the case of your FIRST-PRIMARY TRACKS, [before overdubbing commences], you'd typically have the SUBMIX channels switched to POST-, for your primary inputs. This routes mic/line signals through the MAIN mix channels, and then through SUBMIX. INPUT signals FROM THE MAIN INPUT PART OF THE BOARD are also assigned to BUSSES, of course, and BUSS-OUTS are routed to record-ready & activated TAPE-INs on the tape deck,... as well as the input signals being simultaneously routed through
the 8x2 SUBMIX, for cue monitor. At this point you'd probably listen to the SUBMIX on PHONES, in most cases
THEN, WHEN IN THE OVERDUBBING PHASE,... on those trax you wish to monitor from the TAPE/playback section of the recorder, you'd simply switch the corresponding SUBMIX channel to TAPE. THERE, you have your proper CUE mix. In this configuration, you'd likely be listening to some SUBMIX channels switched to TAPE, and other SUBMIX channels switched to PRE- or POST-,... [doesn't really matter, but I prefer POST-].
With SUBMIX channels selected to TAPE, there is NO EQ inline on the tape signal,... just raw tape signal directly from the tape recorder, and into the SUBMIX. It's just cue, so EQ on the TAPE-RTN signals is not important at this stage. Plus, the SUBMIX section controls the PAN, in this configuration, NOT the pan pot on the MAIN channel section.
Rule of thumb, [IMO], is that you'd be best to learn and work a session or flow of recording sequence, by FIRST remembering to ALWAS TRY to keep the MAIN [INPUT/BUSS ASSIGN] part of the board SEPARATE for INPUTS, and the SUBMIX section for CUE, [listen & playback], especially while overdubbing.
[Repeating myself again:] The SUBMIX provides adequate and needed isolation from the signals on the MAIN/BUSS section of the mixer, which is absolutely necessary when recording and especially overdubbing. This helps in AVOIDING the dreadful feedback loop that FZ & I were talking about. Every owner of an M-30 will sooner or later patch the dreaded feedback loop, and it's a lesson to learn on how to avoid this, if not for your equipment's sake, but also your ear's sake. PLUS, if you use the MAIN mixer section for cue,... which can be done, of course,... you'll be in a more complex configuration to consider feedback avoidance, AND THE UNWANTED OR INADVERTENT TRACK BOUNCE situation.
OKAY, on to the main point: To REMIX through the SUBMIX section, you'd simply switch the SUBMIX channels to POST-, and then switch each of your MAIN INPUTS to RMX [remix, or TAPE-IN]. THEN, you'll be routing the TAPE RETURNS FROM THE TAPE DECK, THROUGH THE MAIN PART OF THE MIXER, COMPLETE WITH EQ, MUTE AND FADER INLINE, and THAT RMX-EQ'd-MAIN-CHANNEL SIGNAL would then be routed SECONDARILY through the SUBMIX section. Submix, in this case, would be more like a "master" stereo section, to use this term loosely.
HINT: In this configuration for mixdown, the SUBMIX LEVELS, PAN POTS and SUBMIX MASTER POT are active, inline to your final mixdown signal. Pan pots on the MAIN CHANNELS will NOT be active in this mixdown example, nor will the MAIN-MASTER-FADER be active or inline, in this example.
PLUS, I'd recommend SWITCHING THE VU METERS to submix to get a handle on overall stereo level and balance of the SUBMIX section, so as not to induce any unwanted distortion in the SUBMIX section itself, inadvertently.
Honestly, that's how I work, for the most part, and you'd probably not be surprised by my summation, to then learn that I use the 4x2 MONITOR section VERY LITTLE. But, it's there if you want it, for a separate and external BUSS-MONITOR mix, if you need it, or, as I said before, as a handy BUSS-EFF-SEND port, if you so choose.
DUDES, those are my current thoughts and explanations on the M-30, it's use and virtues. NOTE: NOTHING I say is cast in stone, as-law, or anything,... and there's probably other good patches, configurations, or creative uses for each of it's sections, that I have not covered or even thought of yet. FZFILE found how his patches and methodologies work best for his sessions and goals. MUCH of this setup and patchwork is situationally dependent, of course.
I will add, in closing, that I've dialed you in on many of these concepts of the M-30's use in studio recording, and you should really read it and then get hand's on with the M-30, to really appreciate fully what I've laid on'ya.
It seems easy to sum it up, and lay it all down in one or two handy memos, but these ideas and concepts I've detailed have come from 21 years of first-person, hands-on use of the M-30, and I'll admit, that I'm cutting you to the chase on things that may have taken me [perhaps] years to learn. THAT's WHY I'm known for saying that there's more to the M-30 in flexibility-terms, than meets the eye. I've been there,... had the successes, and made the mistakes, too.
I guess one great point not to gloss over, is that all this SUBMIX and MAIN MIX switchable flexibility was an ingenious design concept of TASCAM's, from the '80s, that was driven toward the idea of switch-function routing of signals, thus eliminating all the traditional CABLE repatching that would be necessary on most of the other contemporary boards of the time. Other basic boards of the time would require a physical repatching of cables to switch between the main/cue/remix functions, but the M-30 and subsequent Tascam mixers streamlined all that with TOGGLE SWITCHES.
Another relevant point to NOT gloss over, is that there's a DIRECT-OUT jack on every channel, which helps in handy or creative routing situations, no doubt, especially the occasional 8-track-simul-recording to tape,... of which you'll be obliged to use at least SOME D-Outs, because the M-30 is a 4-BUSS board. You get 8 BUSS OUT jacks, but they are 4-groups of 2-jacks, in parallel. I hope that's a clear enough point to make, for your enlightenment. D-OUTs are a handy way to expand the in-out-routing flexibility of the M-30 mixer.
That's a moderately long memo, jammed full of tips and tricks. I hope it's not too long, and that I've been clear enough on all points. Ask me more, and I'll tell'ya more.
That's a lot of techy stuff that's best to be read carefully, and contemplated over some hands-on use of the M-30 board, for it to really soak in & make sense. Hope that helps, & good luck!