Perhaps you have just gotten used to the light output?
With age, they generally get bluer - so in theory there could be less light output, but not by a whole lot.
If you want to replace them, you need a pair of xenon d2s 4300k bulbs to keep it stock. If you want more blue (but slightly less light) get a pair of d2s 5000k bulbs. If you in-fact have gotten used to HIDs and you feel that you now have less light output, I don't recommend going above 5000k at all.
Philips, Osram, etc are great brands to choose from.
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