There may be more I'm not aware of that I'll find out about when I get them, but there are some improvements over the HD3k and some extra features.
Improvements:
1. Better heatsink
2. Fixation rods that help prevent damage from vibration
Features:
1. Digital readout of all your settings, so your crossovers and gain will be spot on. This is perfect for gain matching two amps without fine tuning the knobs and no need for a DMM.
2. Feature that allows you to limit the output power, so you can set the amp up so that it will not clip no matter how much you turn it up. This is the cool feature, I'm gonna set my gain a little higher and use an LC-1, so even songs with lower bass amplitudes can be played at full tilt safely.
3. I believe the crossovers have a lot more options, I will confirm this when I get it though.
4. Voltmeter on the heatsink of the amp.
And I'll make sure to do a review a week or two after I get them in.