Bass head repaired!
A year since my Trace Elliot bass head went pop I decided to fix it as my replacement Laney head doesn't compare to it. After hours of diagnostics and drawing up schematics of the power amp board I discovered that the fan power supply feed and the P-Channel MOSFETs were damaged. I removed the fan supply components and took a seperate supply from elsewhere on the board. I tested the MOSFETs using the technique detailed in my electronics testing section and found that both of the N-Channel MOSFETs checked out fine and that both of the P-Channel MOSFETs were blown. I ordered some replacement FETs, checked all of the other components, re-soldered some suspect joints and reassembled the power amp board. I tested the amp again and it sounds like new! Some switch cleaner on the potentiometers and a good service has nicely finished off the job.
Bass head went pop
My old Trace Elliot bass head that I had managed to repair decided to go pop today! I had just finished installing additional fans and temperature sensor to keep and eye on it and during testing it died. When I tried playing through it there was lots of crackles! 
I've had to bite the bullet and have ordered a new Laney head.