Do I need an amp?

55 ohms should usually work on a typical on-board Realtek DAC, I think. It's maybe on the upper limits for a typical on-board Realtek, but still.
I think, I did mention to you earlier that AKG usually goes for more flat (accurate) sound than boosting lows. Especially if you're editing sounds you want accurate sound like that. You can "fix" it by either tweaking the equalizer in Windows or use a headphone amp to add the sound signature you prefer. I personally have a tube amp connected to my PC, as I prefer the more fluffy sound you get out of those. Mine is a Bravo Audio V2. Fairly cheap, but be warned that cheapness is mostly due to the tube they bundle is pretty bad, so that has to be replaced, too. \o/ A decent transistor-based headphone amp can be found at around €50, I think.

Another alternative is a slightly beefier audio card, like an ASUS Xonar DGX, with comes with a more powerful amp for headphones. Those you can get for around €30-40.

One more thing: The lower sound level you get with the AKG's vs your old ones is quite natural. More ohm's means you need to raise the volume more to get to the same sound pressure.