IMHO get the 500, with that radio you do not need the driver in the 667, your radio will work fine with the 500.
You have enough radio to get the Max power out of the 4, 2879`s with out feeding extra current to the drivers.
The Dial-a-watt uses a 2 watt variable, pressing the green dial-a-watt button also inserts a 75 ohm 4 watt metal oxide resistor "pad" for protecting the variable.
More often than not they get dirty, it is a small carbon track inside and they just do not hold up well twisting them back and forth, if you are buying new, skip it and get the 4 button 500 and use the radio to control the drive power.
If you are buying used and it is a V model, just run the amp with the red button in and again control the drive with the radio.
For SSB just use the Yellow delay button.
73
Jeff