As you probably know, the Bose 901 Active EQ is there to modify the frequency spectrum fed to the 901's so they will produce a flat response. Without it, they sound acceptable in the midrange but lacking in bass. Since they have no woofers or tweeters, the modification is drastic and should never be applied to non-Bose-901 speakers or they could be damaged.
This setup should work for mixing and playing but I'm not sure if recordings directly from the unit's REC Out or Line Out would have the Active EQ effect on them. You'll need to perform a few tests to see how it all works for playback AND recording.
Remove the jumpers for Pre In >> Out
Insert the Active EQ in their place by running:
1. RCA cables from Pre Out to the Active EQ In (From Aux, Tape Out)
2. RCA cables from the Active EQ Out (To Amp, Tape In)
Use ONLY 901's on this amp with the EQ.
The Bose Active EQ has its own Tape Rec/Monitor loop to replace the one it normally occupies in a typical receiver application, so if you wish to record your mix you should use those connectors for your tape deck and there will be no problems with the Active EQ effect being recorded. The Monitor Tape/Source switch on the EQ functions just like a typical Tape Monitor. For playback of the recording, switch it to Tape.
The manual, available at http://lib.store.yahoo.net/lib/acekaraoke/MA3000K.pdf
doesn't get into any detail on the innards of the mixer.
In a typical integrated amp or receiver, the jumpers removed are between the Preamp Output and the Main Amp input, but BEFORE the amp's volume and tone controls, so manipulating them would not affect a recording. You should experiment with it by making recordings through both and adjusting speaker volume to see if it changes.
I would be curious to know what the functional difference is between the designated LINE Out, REC Out and PRE Out jacks in that regard. Normally, Line Out and REC Out will not vary with amp volume, but I expect in this mixer it does, or you wouldn't be able to control the volume from it. Experiment.
Of course, you probably want the mixed effects to be recordable, so I assume they'll appear at all outputs. Play back the test recording (at an initial low level, just in case) and see if you can detect an exaggerated low end (due to the 901 bass boost being applied in recording AND playback through the 901's. If the recording volume doesn't vary with speaker volume and the playback sounds as flat as the recording you're good to go.
If there is are volume variations or an additive effect, you may have to draw the REC Out to the deck's input and run the deck's output to the Tape In on the 901 Active EQ, monitoring only on deck playback.