As you've already figured out Karel your 9:1 transformer works best when the antenna presents a 450 ohm load at the frequencies of interest, (so requiring a specific antenna length).
The problem with just simply using a 'tuner' is that you have to account for losses in your feedline and most get around this by utilising a remote tuner at the antenna end to avoid such issues. Others (like myself) utilise ladder-line feeders to circumvent such losses that would be presented with coaxial cable but even then it isn't a magic cure-all often requiring additional or less feedline lengths to avoid extremes.
The last thing you'd want to do is burn out your MFJ tuner when presented with such extremes a mismatched antenna can present nor introduce massive losses on your feeder making your actual antenna fairly useless. You'll also want to avoid common mode current issues and the easiest way to get around that is the introduction of a 1:1 common mode choke between your transceiver and ATU.
Try to think of your antenna experiments as a 'whole' antenna system with everything from the back of your rig to the tip of your antenna being inclusively important.
If this helps any I'll provide a link (although to a commercial site not endorsed by myself or CT) that provides some additional information when utilising antenna impedance matching systems :-
https://www.hfkits.com/antenna-tuners/
There's much more information out there besides and worth exploring more but you may find yourself having to trawl through misinformation, myths or hype plus it's always best not to make quick assumptions of your own.
I wish you all the best with your antenna experimentation and although it can be frustrating at times just ensure that you're enjoying yourself.
(I think I've had more fun with antenna experimentation since becoming a Ham than anything else!
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73
Victor
Today at 2:33 pm by 53 Prefect
Subject: 1:9 UNUN, why? 
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