I've not heard your GK amp but I bought the ampeg head a few weeks back.
I've had class D amps before, the SWR and Genz and found them to be too clean with no soul. I then went back to good old linear amps with all the weight problems but with a better sound.
Now my back is getting old and tired so I was forced to take a second look at class D amps. I tried the Ampeg thinking I would hate it as I have never been a fan of Ampeg except the mighty SVT.
The sound is very transparent without the sterile sound I found with the SWR and Genz. It has a warm punchy sound.
If you drive the input into clipping it does not sound very natural , it breaks up like a broken drive unit. Best to use an out board effects unit if you want it to distort.
I really like the compressor as it works well and has a sound I really like. At high settings it allows a lot of the start of the note through and then clamps it down, it's not transparent but has bags of character.
The eg is good. It seems to do very little until turned passed the 3 o'clock position then comes in very strong. I boost the bass to 3 o'clock, mid frequency set to 800hz with a little boost and top set flat. Perfect for a P bass. For a jazz I just punch in the bass and top boost switches which adds bass and top with a lower mid frequency cut.
I was also very impressed by the 1x15 portaflex cabs, these sealed cabs partner the amp really well. They have a similar sound that compliments each other. Warm with punch.
Now for the good bit.... Cost.
I can't believe the cost of the amp and cabs, around £350 each. I have owned very high end and expensive kit from glockenklang and the excellent Warwick Helborg rig and find the Ampeg to punch well above its weight in both sound and cost.
I'm very impressed.