I use a set of four of these wheels to transport my 'TheBox Pyrit 212 Sub A' and cannot fault them! These wheels are sturdy enough to easily support the 45kg Sub and more, as with some padding I strap my two EV ZLX 12p's to the top of it, to make it easier to transport in less loads.
These wheels, when first unpacked, may not feel like they will run smoothly, as in if you were to roll them in your hand they may feel like they are 'sticking' slightly, but as soon as any weight is put on them, they run as smooth as butter!
Overall, a very safe bet if you are looking for wheels for almost any purpose, but remember these are 80x60mm spaced so make sure your sub/other product will support them!