I had a Beta 50 back in the 80's. The bottom tone button defeats the bass and treble adjustments and sends out a flat signal. If you want the tone controls to work, press one of the clear tone buttons for the line you want. The processor A and B are defeatable loops for equalizers. If you use an eq, select either A or B and press the clear button for the line you want to affect.
The CX is an ancient and unpopular form of anti-hiss sound processing that was recorded into some vinyl albums. For this to work, the album must be recorded with the CX info and they are hard to find. I recall having a Spyro Gyra Morning Dance that was recorded in this format. Oddly enough, the system worked well but it also coincided with the emergence of the CD so it became obsolete along with it's recording medium.
I'm not sure if you've hooked it up yet or got it working, but essentially the buttons follow the signal path. You have two lines, each with a selector for it's own source or as tape monitor. You can also blend the sound output from the two lines, which makes it ideal for a kind of DJ preamp. The top row of buttons, including tone, A&B and CX, selects those functions for line one. The middle row sets those functions for line 2. The bottom row defeats those functions.
I haven't had that preamp since I sold it to my brother (along with my Bryston 4B) following my divorce in 1990. I'm only working from an imperfect memory but I'll try to answer any other questions you have. One small note, I did find that there was a lot of cross talk between sources so you may want to make sure that only the source you are using is on, otherwise you will probably hear any other sources that are on in the background of whatever you're listening to.
