Thank you for your reply dirtyferret.
Before I had those, I had some Creative Inspire T3100, the volume button was broken and I kept them on 24/7. The sub wasn't soo hot.
The logitech sub isn't so hot, I just feel a light heat from the pannel.
It's completely fine as long as it's not physically burning your hand on touch. You're putting a lot of power into them, and that in turn creates heat.
If it starts smelling, then there may be an issue.