I have used fixed bias in several 18 watt style amps and the Lightning isn't all that different in the basic power amp. All it takes is a pot (optional) and a few resistors and caps. You can can see a circuit in the Vajra build under DOWNLOADS/MODS on the
http://www.18watt.com forum.
The difference in sound is mostly in the low end. The amp becomes tighter and has a less loose low end. The Vajra build has a switch to switch between modes.
Another place to look is at the Marshall Park and 20 watt amp scheamtics. One or two of those used a fixed bias arrangement.