If purchasing the CA-2010 local, test --- besides the balance -- at the very least ALL MODE selector options: stereo, mono, etc. and Input Selector inputs: Aux, Tuner to make sure they are working. Or at least to know how much cleaning may be involved.
Check the rear connections to see if wiggling them causes drop outs. This may indicate bad solder joints. And it's not easy to access some of the boards inside. In my case, I have just been touching or wiggling them in the back whenever my right channel drops.
I believe the manual says don't be flipping B to A if over 30 watts when on, but so you don't give the seller a heart attack ask him to demo switch for you or turn off then select Class A. Until I get more ambitious or tired of that. It's very subtle and you have to be listening for that clearer sound. Myself I seldom switch to A just because of the heat issue, but it gets pretty warm with B too so you need good clearance in a cabinet and or no stacking on top.
After that, you should be very happy with a working CA-2010.