This, read this.The difference between 400 and 600 watts is ~1.75dB -- only marginally perceptible, IME. You've got a good amp... I wouldn't sweat it.
If you're absolutely determined to squeeze out that perceived last bit of performance, you'd be better served investing in a 4Ω cab.
Your new amp will be slightly less louder than turning your current amp’s master volume control up one click.
So unless you are hitting the limiter in your current amp, just turn up.

