How much power the amp puts out for the sub channel in your case will be dependent on the SUB that you have. if it's a 4 ohm sub, it'll put out a certain number of watts, but if it's a 2 ohm sub it'll put out more power to the sub (but it'll be working the amp harder in the process and generating a little more heat).
For the money, I'd go with the PPI amp over the kenwood. the kenwood is almost double the price. The PPI amp will put out as much or more than the Boston amp you're replacing.