Board repair for subwoofer

I wouldn't rule out replacing the sub with a new one. The amps in your sub are matched. If you go with another amp there will be a mis-match. The two speakers will no longer couple properly. 

For all the time, money and effort a new sub seems like a very reasonable option. For your needs a simple SVS or Rel may be a far better choice. 

@gdaddy1 

Yes, I would have to get an amplifier plate that will replace the current entire plate, which has two circuit boards in it, coupled (each with an amplifier for each driver (woofer)).  I’m sure there are options out there, as running one in parallel mode, as a Y branch, as @devinplombier suggests. ..but replacement may be a last option if there is no reasonable solution to these cheap amplifier boards on Martin Logan subwoofers (although perhaps a couple of my vinyl records also plays a role in burning up a component in these amplifier circuit boards.)

Would you recommend the class AB over the class D for subwoofers? (Hoping not to open a can of worms.)

A fair number of subwoofer plate amps are / were Class AB, but they tend to run hot when confined inside an unventilated enclosure, leading to their early demise.

At this point, most people agree that Class D does an outstanding job in low frequency applications. I wouldn’t hesitate to recommend Class D for subwoofer amps - all the more so if it’s a plate amp, due to Class D’s lower operating temperature.

If you have a high-quality subwoofer and you love its sound signature I would recommend converting it to external amplification, due to the added flexibility and elimination of the environmental challenges inside enclosures (temperature, poor ventilation, vibration).

@drbond what is your sub's model number?

@devinplombier It definitely sounds reasonable to try external amplification, especially if I can’t get the bad board repaired.  I have the Martin Logan Balanced Force 212’s.  (Dual 12 inch drivers in a sealed cabinet, with each driver powered by a 850W class D amplifier.  So there are two boards attached to one plate.)

You have very nice subs, there has to be a reason the amps keep popping. If you do a google search, that doesn't really come up with them.  Are they getting good clean power? How are you hooking them up? What gain are you running them at? Is it the same sub, or has it been both? Are you using the factory power cables?

Usually it's not hard to spot a component failure from heat/over driving. The board will show browning, or heat damage, usually the component will also show the same type of damage. 

Class D amps are perfect match for subs, lots of power/current, small footprint, low heat, low power consumption.