The 45000 allows you to overdub onto the Mixdown track in a similar fashion to how you overdub onto the four main tracks where you add new audio with each pass of the loop. The best technique for adding more tracks to the Mixdown track is to use this overdub feature. Here are some tips for building up and adding to the Mixdown track:

1. Fill up the four main tracks. I assume the Mixdown track is currently empty.
2. Push the Mixdown slider down to its minimum position. Press the Mixdown slider so that it’s LED is lit which means it is now the active track.
3. Press the record button (both the REC and PLAY LEDs should be lit). You are now mixing your 4 tracks to the Mixdown track.
4. After you mix at least one full pass of the loop onto the Mixdown track, press the RECORD button to put the 45000 into PLAY mode, push the Mixdown slider up to maximum and mute the other 4 tracks by pushing them down to their minimum positions. You are now listening to the Mixdown track. If you are not happy with the Mixdown track, go back to step 2. If you are happy proceed to step 5.
5. Record new audio on the 4 main tracks while listening to the Mixdown track. At this point there is already audio on the 4 main tracks so you will most likely want to engage PUNCHIN mode while recording the first pass of any of the four main tracks.
6. Once all four main tracks are filled, they can be overdubbed onto the Mixdown track. Make sure the MIXDOWN slider is at maximum, set up the 4 main tracks as you want them. Press the Mixdown button so that its LED is lit and then press RECORD to record the new material onto the Mixdown track. On this step it is important that the MIXDOWN slider is set to maximum because its slider sets both its volume and the overdub amount. If the slider is below maximum, the previously recorded audio on the Mixdown slider will increasingly be reduced in volume with each psas of the loop.
7. Once the second Mixdown is complete, go back to Step 4 and repeat the process as many times as you like.

Step 2, set Mixdown slider to minimum is correct. That way you don’t have to be careful about when you start and stop recording the Mixdown track, as long as you get at least one full pass during Mixdown record.

The manual is suggesting that you connect the 45000’s Left and Right output jacks to its Left and Right Input jacks with 1/4″ instrument cables. So you record the output of the Mixdown track back into the input of the 45000. You have to be sure the DRY slider is at minimum so it does not introduce positive feedback and the 4 main track sliders should be at minimum as well. If you record onto just one of the main tracks, it will convert the Mixdown track to mono. If you do a stereo recording then the Mixdown track remains stereo.

There could be some loss of audio quality by bouncing via cables but if the gain structure is set up correctly, it shouldn’t be too noticeable. Personally I think you are better off leaving the Mixdown track where it is and overdubbing on top of it with new tracks.

Regarding the placement of the Mixdown slider in step 2, when it is set to minimum, any audio that was previously on the Mixdown track is erased as you record onto the Mixdown track. The new audio that is recorded is not effected by the Mixdown slider’s setting.

The reason you might want to set the Mixdown slider to its minimum position, for your first Mixdown recording only, is so that you do not have to be careful about where you start and stop Mixdown recording. If you happen to record 1.5x the loop cycle during the initial Mixdown, the section of the loop that recorded twice onto the Mixdown track will not jump up in volume, it will still sound like you mixed just one loop cycle. If you did this with the Mixdown slider set to max, the section of loop that was recorded twice will jump in volume on the Mixdown track because it has been recorded twice.

After you finish your initial Mixdown recording, move the Mixdown slider to its maximum position.