I think,...
You should mix your parts from the 12/4, as primary input mixer, to a 4-buss mix that is:
Buss 1, Kick mic
Buss 2, Drum kit mics
Buss 3, Bass
Buss 4, "scratch" vocal and guitar
Cable the 4-buss outs of the 12/4 to the 1/4" inputs on the Tascam, input Channels 1-4.
TRIM to the LINE position, Channels 1-4.
Record in DIRECT MODE on Tracks 1-4.
The "scratch" vocal and guitar gets recorded to track 4, and the idea of "ghost" track, is a dead issue. Commit the "scratch" parts to tape at the outset, and rerecord them later, if you like. It won't hurt anything.
Why be obsessed about routing stuff through the mixer that you're not recording? It's counterproductive. You're making yourself crazy with it. Routing parts to Buss 4 to *not* record them, seems irregular, but you could do it if you really wanted.
Just put the Tascam's Track #4 mode switch on SAFE position, and go about your recording. The scratch track, Track 4, will not record the "ghost" channels of voice & guitar, or whatever else was on BUSS 4, on the Mackie.
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The 424mkII's headphone & LINE OUT feed w/Rolls would be suitable for monitor, also the Mg 12/4's headphone could be used for monitor.
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The bouncing technique you describe,... is doable. Not to say it would improve the sound, though. Any bounced set of tracks presents compromise of the sound.
You can improve your track count with moderate use of bouncing, but I wouldn't get over zealous with it. 10 tracks on a 4-track is feasible, through the used of a collapse-bounce or three, but it will likely carry with it a lot of sonic compromise, or degradation of some sort.
Not to say bouncing tracks is not a fun and useful technique, it is. Just not to overdo it.
Plus, you'll have to mix as flawlessly as possible, and commit parts into the bounce mix, choices you will have to live with, for ever. That's what I mean, by compromised in the mix. Not to mention, if your bounce mixes are not nearly flawless, your tracks could turn to mush, when it's all summed up.
There's something about planning strategically what parts you're going to bounce, for maximum impact of the technique and minimum generational loss or noise. Remember, once you bounce parts together, the individual parts are fixed in the mix. There's something about technique of bouncing tracks that works, versus bounced tracks that turn your mixes to mush, that you just have to learn by doing it.
As far as bouncing goes,... [MO],... I've done it,... to excess sometimes,... and often times when it worked nicely, but it never worked when I over did it, and it usually worked best when I took a subtle, planned approach to bounced tracks. I don't look at the "full collapse" bounce [10-bounced tracks on a 4-tracker] as practical, but I'm more inclined to bounce a minor part in behind a more major part. 4-trackers often force the issue of bouncing.
I'd generally not like to bounce tracks at all, if possible. If more tracks are needed, I'd prefer to move up to a larger format recorder, if possible. That's just me, though.
Anyway,... before I ramble further.
