Welcome!
Since you found two amps I'm guessing your dad used the bigger amp (1.5t) for the 'main' system and the 200t for a second room? But if you're only working with one room, the M-1.5t should do the job nicely. But either should be fine if it meets your power output needs.
For the volume control, in some cases it's hard to get the liquid into the pot for cleaning. If the CRC doesn't change at least reduce the crackling, I'm guessing it isn't getting into the wiper of the pot. If you are comfortable with removing the cover so you can get direct access to the pot, a straw on the CRC sprayer should be able to get the cleaner in there. Look for a little opening/notch in the pot body to allow access. Spray it with power off (and the system unplugged and sitting for 5+ minutes), rotate the pot fully back-and-forth 4-5 times, then reconnect everything and try again.
And @Balok had a good insight as the Balance pot can also get distortion into either audio channel. In fact, if you've gained access to one pot, it's a good idea to clean them all.
Good luck.