The AX-20 is a crude 4 into 2 switcher which does not allow panning. Only left, right or mono switching. So, if your needs are
extremely basic, that box will allow you to do a transfer down to a 2 track deck.
But ideally, when it comes to overdubbing and being able to create acceptable cue mixes while doing so, there really isn't a replacement for a dedicated recording mixer which will allow you to have proper recording levels to feed the tape and separate cue mixes that help you to properly hear the parts that are both live and going to tape balanced against what's already on tape. As well, a recording mixer will allow you to do far more elegant mix-downs which employ panning, adding eq and external signal processors such as reverb and compressors, noise gates and whatever else floats your boat, sound effect wise.
The monitoring facilities on the 3340 were not set up for this all on its own. And if you've ever had a peak at the owners manual of it, you will read that TEAC confirms that and suggests strongly the purchase and usage of a recording mixer to complete the package. In that era, TEAC offered the M2 and M2A mixers but any of the newer 4 buss recording mixers can also be used and will offer cleaner specs and more routing options.
Keep in mind that the 3340 was designed for two different end users. The fist was the hifi enthusiast of the day, who wanted to listen to quadraphonic recordings from popular artists of the day, (1970's) and it was also aimed at the aspiring musician who wanted to dabble in the art of recording live music and not rich enough to be able to go to a commercial recording studio.
Know your gear. Know what it was intended for and you'll be that much further in the picture.
Cheers!
