I haven't used the QP-16 but I did just add two QP-9s to my 6' tank. They haven't been on the tank for very long (less than a week) but I like them. I have mine in alternating waves modes and there is an audible whistle, I'm hoping it will stop once they break in more.
For what they do and for their price, I quite like them.
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Yup still whistling, it's at that pitch that just irritates you.
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