It doesn't matter what the amp is rated at, it could be a million watts, it's the volume you're putting through the cabs that counts. So, just because you want to use a higher rated amp why will that make you play it louder than your current amp?
You don't say anything about running out of power with your current amp, so given that most people who up rate their amp will stay at the same volume, the extra watts will only give you more headroom, which will get you better tone and give the amp an easier work out in the process.
Anyway you were probably running your 600 watt amp at a lot less than half power so your cabs won't be in any danger.