1. What is a "XCPC"? Do you mean an XPCP? If so the XPCP does not amplify, it only couples the X10 command from one 120V line in the panel (where the command originated) to the other 120V line. It does not help if you have a noise problem (or nasty signal sucker). IMHO in todays world of switching power supplies, CFL, and LED bulbs, (all noise makers) a XPCP is useless. A repeater coupler at least amplifies the X10 command on both lines.
Did you trouble shoot for a noise problem? If the CM15A worked at one time but either degraded slowly (could be a noise source on a breaker circuit other than the circuit the CM15A is plugged in to) or quit suddenly (could be a [new] noise source on the same circuit the CM15A is plugged in to) you need to trouble shoot for noise before assuming the CM15A has failed (not saying they don't, but if it still talks to the computer and shows activity on the Activity Monitor I would look for noise or signal sucker before assuming the CM15A is bad.
Plug an Appliance Module into same outlet as CM15A, does the Appliance Module work then?
2. Yes a double pole 220V breaker connects to both 120V lines in your panel (in north America). By code your XPCP should be connected to it's own double pole 220V breaker, but as a test you can temporarly connect it to a 220V breaker for the clothes dryer, water heater, or stove. Do not connect it to a 220V breaker for the HVAC since the compressor and fan motors can attenuate the X10 commands.
Try searching the forum for "noise" for better information and examples of noise generators (and signal suckers) other users have found (cell phone chargers, CFL and LED bulbs, computers, UPS, flat panel TVs, etc).
X10 is long on tooth, but can still be made to work, it's just becoming more of a challange which takes some patience and step by step troubleshooting.