It sounds like your HD600 might just be reaching its limits, especially with the bass-heavy sound, which could be causing that crackling noise. The HD6XX is quite similar, so if you're on a budget and looking to get a backup, it might be worth grabbing a pair while the deal's still available, just in case. However, if your HD600 is still mostly fine, you could wait and see if the issue persists. If you decide to upgrade, the Speedball Crack is a solid choice for pairing with your headphones to improve overall sound quality.
Ultimately, if you find a good deal on the HD6XX, it could be a smart move to grab them now and store for later, but you don’t necessarily need to rush unless the issue with your current pair gets worse.