You can get bags that are unfilled and then fill them in then.
Take it down and work some ground and pound stuff, redistribute the filler material a little ;)
Or you can just add more material, old t-shirts work fine, as it settles. That won't soften the bottom up, but will keep the top from getting to soft.
I have a Ringside freestanding heavy bag. I don't know how they stuff it but that doesn't seem to be a problem. I was thinking that the OP might get a water filled bag. We have one in the gym at work and it's not bad.
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And is there any major reason to go with a more expensive bag (aside from the fact that it might last longer) ?
It depends what you plan on using the bag for: personal use or gym use.
As far as I know, any half-decent leather heavy bag will out last you and possibly your children. Don't fuss too much about looking for an "extra durable hide".
Presumably there'll be a lot more people pounding away at this thing than if you had it in your basement, so probably a better idea to think about it a little more in terms of budget.
Looks like a great bag. They don't go much into construction, but it looks decent.
The one thing I noticed on the Powerhide MT one was that it had the 1" foam, so you don't have that 'Rock friggin hard' bag, aka save the hands/wrists.