Boy ... that's a tough one.
Yes, the mixer is a "few" years older than the Portstudio: about 20. I think it came out in 1976 or so.
You could:
- Use the Model 3 as an extension (or submixer) when tracking, so you could record more inputs. I think the Model 3 has 8 mic inputs (yes?); the 488 has 4 (two with XLRs, two 1/4" only) + 4 line ins + 2 stereo line ins. So you could hook up 12 mics and a quite a few synths or drum machines, or line outs from amp modelers, or something, and record them all at once. You can only record to 4 tracks simultaneously, though. Do you have 12 mics?
- Use the Model 3 mic preamps in place of the 488 mic preamps, by running into the line-level ins on channel 4-8 of the 488. I'm not sure if there's a good reason to do this. The Model 3 preamps are, presumably, somewhat different from the 488's. Whether that's a good thing or not, I couldn't say.
- Use the Model 3 to mix down. Unfortunately, this is sort of crazy. The 488 has no direct tape outs, so you need to kluge up a way to get the tracks out* ... once you do that, you might as well just use its mixer.
*e.g.: track:
1 from channel insert
2 from channel insert
3 pan left and assign to 1-2 bus, take from line out L
4 pan right and assign to 1-2 bus, take from line out R
5 pan left and assign to 3-4 bus, set monitor to 3-4, take from monitor out L
6 pan right and assign to 3-4 bus, set monitor to 3-4, take from monitor out R
7 from aux 1
8 from aux 2