When buying a D2 the "modded" units always fetch more on the second hand market. Earthquake mod them for free at World finals, and will mod them of you pay them, and thats what makes them rare and more expensive.
Or you can do it yourself. There's a lot of mods out there for the D2, here's photo's and part numbers of the most common ones that really give you more power !
Replace R08 with 10ohm
Replace R37 with 75kohm
This will increase rail voltage for more power output.
Anti-feedback on HIP4080 output side IC. Use Zener diode 1.5KE68A between Pin 18 -> 20, Pin 4 -> 2.
This should improve sound quality during musical playback.
Some people say the only real mod for the D2 is to replace the factory 20A fuse with 30 or 40A rated fuses. This is the only mod required to run the D2 at 0.5ohm for SPL.
Three other mods I am aware of for the D2 involve the internal capacitors:
1. Replace the 10x OEM fitted 16V capacitors on the input side with 25V capacitors of the same capacitance (older D2 models).
2. Replace the 2 filter capacitors on the output side rails with single or doubled units (parallel) that are larger (physically).
3. Add 2 additional capacitors on the input side, there's points next to the 4 center caps to add two more of the same rating.
None of the capacitor mods yield any SPL benefit, but do look cool for when trying to sell the amps.