B22 outputs a clean 18W into 8ohms and you only need 0.1W, so it is fine as is, with stock heatsinking. I have special heatsinking and it can get painfully loud with my speakers. Balanced B22 will output over 50W and I've tried that for speakers, too. I did not use a Zobel.
The other way around, using speaker amps to drive headphones, works great too. I've used UcD400 modules with a regulated power supply to drive headphones. Like I said, I wouldn't worry about driving 2" or 3" inch 8 ohm fullrange drivers. It's easy peasy.
The other way around, using speaker amps to drive headphones, works great too. I've used UcD400 modules with a regulated power supply to drive headphones. Like I said, I wouldn't worry about driving 2" or 3" inch 8 ohm fullrange drivers. It's easy peasy.