you have 5 speakers AND a sub, correct? That makes 6 speakers.
The 5 speakers plug into the receiver with 2 conductor speaker wire, right?
Does the sub plug into the receiver with the same two conductor speaker wire also?
You will need to buy a powered sub. The other 5 speakers are fine.
Your sub doesn't have a built in amp.
From what you describe, apparantly, the HTIB had a sub amp built in, which is not the norm.
The watts and ohms of powered subs are immaterial to whatever receiver you buy. Don't agonize too much over specs when picking out a powered sub. Just go for the best you can afford.