Roger at RC has been exceptionally helpful for me.

I can confirm the 24 v jumper idea, I did it by accident for a couple months. Amazing flow (it's overdrived) but it does get quite noisy compared to 18 v.

I got new style wavebox with 4 6105's, one of the pump had a motorblock issue when I first got it but after a few emails with tunze Canada they replaced it with a new motor block. Beside that, every thing works perfectly. I have recently purchased a new 6105 but it seem to be a bit noisy, I opeded it up and clean it,(not that there is anything in there) but it has helped quiet it down. It must have been something in between the prop and the motor block, but who knows. If I were you I would email tunze Canada since they will be the people who are going to replace the defective pieces for you. I think the company that supplies tunze in Canada is Xenia.

No problems with my used 6215..works great.

When I got home from night shift this morning I did a little troubleshooting... and fragging. It took me a few tries to get the 6215 running again, but it seems that the problem may have been that the impeller wasn't in quite far enough...? Seems to be good now. That said, I'm not sure why it worked for a month and then stopped, but I'll keep my eyes on it and see if it acts funny.