The stocks with chains isn't working for me with the snowblower attachment.
What exactly is not working?
What is your property like?
How steep are the areas where you are having issues?
There are some here who swear by turfs, chains, and weight. But, that requires LOTS of weight, probably much more than you currently have. I think the X300 series recommends only 2 42lb rear weights. You will need to supplement that with wheel weights and probably loaded tires. This is assuming that traction of the rears is the issue you are seeing.
The problem with the X300 series is it has no diff-lock, so the tire with the least traction is the one that limits the motion. I think the X300 makes an excellent mower, but I don't think it is the best for traction challenging locations or tasks. The X500 series gets you diff-lock, plus more base weight and the ability to add even more weight. That would help a lot. The X700 series gets even more base weight and capacity. It is also available with AWD.
My X749 really impressed me with its capabilities in light powdery snow with a blade. Based on my testing in 5-7", I would estimate it would have still been unstoppable with at least double the amount. However, the next week, when conditions and the snow were very different (heavy and wet), it too was traction challenged. I had to modify my technique significantly in order to complete the job. This was more in line with what I was expecting. Having driven 4WD and AWD vehicles of different sizes in different conditions over several decades, it comes down to understanding the capabilities of the machine you have and how much you can improve it with certain changes. I don't believe HDAPs will be the change you want.