Don't have a WP-xx as yet, not a hater, and don't want to hijack this thread, but to rather to reinforce this post and offer a solution to low flow.
I find I take my Koralia's out every 3 weeks and scrub with hot water and toothbrush. I once had an issue with an impeller wear, and Tech Support at Koralia suggested NOT using vinegar due to corrosive nature. They sent me a bag with cleaner (along with replacement impellers), which worked, but I have not reordered.
So my 2 cents is, this happens to most if not all power heads, and just keep the units clean (as part of normal tank maintenance).
Sincerely,
David