Let's see if I can tag on to what has been said without saying too much and confusing you.
I really want to help you get comfortable.
If you typically record 1 track at a time, then the M-308 is more than everything you need.
It has direct outs on each of the 8 channel strips so if ever you
did want to record all 8 tracks at once you could do it no problem, though you'd have to plug the 8 inputs of the Model 80 to the 8 direct outs.
Normally it sounds like you would be just having the 4 program "group" outputs connected to the Model 80 inputs. As was mentioned, the M-308 group outs have two jacks for each output, so outputs 1~4 would connect to Model 80 inputs 1~4, and group outputs 1~4 would ALSO be connected to Model 80 inputs 5~8. They could stay connected all the time since there are two output jacks in parallel for each group output on the M-308. This is a perfectly normal way to do this. Often times a 4-group mixer is connected to an 8-track machine...and often an 8-group mixer is connected to a 16-track machine. There's nothing wrong with an 8-group mixer to an 8-track machine of course, and neither is there anything wrong with a 4-group mixer connected in this way to a
16-track machine. Program groups or "subgroups" are simply a convenience in a recording setup...like an infinitely variable patchbay.
Understand this: each mixer channel 1~8 (on the M-308) can be routed to any one or all program groups using the ASSIGN switches on each channel strip along with the PAN control. Let's say you connect a microphone to channel 1 and you ASSIGN it to program group 1. Using the dual output jacks on the M-308 the mic is now connected to Model 80 inputs 1 and 5. You can now control which track records by using the record arming switches on the Model 80. So you could record that mic on to track 1 or track 5 or BOTH...right?
So what if you want to always have that mic connected to channel 1 of the mixer but you want to record on to track 3 next and then track 6 after that? Simple. Using the ASSIGN switches just route that mic to group 3. Now the mic signal is present at the Model 80 inputs 3 and 7. Record enable track 3 and record away. Next ASSIGN the mic on channel 1 of the mixer to group 2. That mic signal is now going to inputs 2 and 6 of the Model 80 right? You know what comes next...record enable track 6 and record. So you see, in this way you can have sources permanently connected to the 8 channels of the M-308 and each of those 8 sources can make their way to any, all or any combination of tape tracks using the ASSIGN switches and the program groups. You would simply need 8 RCA --> RCA cables or an 8-channel RCA --> RCA snake to connect the 4 pairs of program group outputs of the M-308 to the 8 inputs of the Model 80.
So now what about outputs?
An "inline" mixer doesn't mean a mixer with both mic and line inputs on a channel. "Inline" describes a mixer that has a mixer within the mixer...on each strip, looking at one channel, you can see a main input and one or more ways to
simultaneously access another input and the controls are physically
in-line rather than being separated out to different sections of the mixer. The M-300 series mixers are some of my very favorite. Each channel strip has a mic input and a line input. Each of those main inputs has a separate TRIM control. You can access the line input via one of the aux channels while having the mic input sourced to the main strip...a little sub mixer. Here's the kicker...the M-300 mixers ALSO have 8 TAPE input jacks. Those can be monitored in a separate monitor mixer that is over the group faders so you as the operator can hear any and/or all tape tracks along with the sources of the channel strips...you can independently control the level of each tape track as well as control placement in the stereo field. Those tape jacks
can also be the "line" input source at the channel strip if there is nothing plugged into the LINE input jack...tape return jack 1 goes to mixer channel 1 and so on. What this means is that the tape outputs can, with the push of a button, be the source of the mixer channels for mixdown time, or can be routed to an aux send with that inline "mixer within the mixer" feature for a headphone "cue" feed/mix...again with the push of a button. Very powerful. And remember that there are separate trim controls for line and mic inputs so you can even set and forget your mic trims if you have sources permanently connected to the mic jacks. you don't have to mess up the trim level when you switch to the "line" source (and remember that is the tape returns if nothing is plugged into the line jacks on the channel strips) since it has its own trim pot. One more 8-channel RCA --> RCA snake and your Model 80 outputs are permanently hooked up to the mixer...and can be monitored, setup for a cue mix or routed to the strips for mixdown all without unplugging or plugging in anything.
THAT'S what you are looking for. I would watch that M-308 auction closely. Not only are they a full featured mixer with a relatively compact footprint (but not TOO compact if you know what I mean), but they sound great too.
I sincerely hope that helps and don't feel shy about asking more questions. We love this stuff, and there's no question too simple...that's how we all learn and we learn more by helping and answering questions too.
