I doubt that the dark blue filter will improve things at all.
Your Uputronics preamp already has a filter and it is a narrower bandwidth than the dark blue filter.
Have you tried changing the gain of your setup?
The blue dongle has a preamp built in and with your uputronics preamp as well you may need to reduce the gain for optimum performance.
The number of tracks with just a single message is really high. That would suggest you have a lot of noise on the signal into the dongle. What power supply are you using? Also how long is your antenna cable? You would achieve better results if you moved your preamp to the antenna end of your feeder cable.
Maybe the single message rate is not that high after all. My main receiver is an Airspy mini and that shows virtually no single messages. However I have just reinstalled graphs on my ProStick receiver and it shows a similar proportion of single messages to yours.
So, about 1.2dB feeder loss. Nothing to really bother about then. I wonder though, if there is noise being picked up directly into the dongle from your cables that are almost touching the dongle. It might be worth trying to move the cables as far away from the dongle as you can, to see if that improves things.
Do you already have the dark blue filter?
If you do, as an experiment, try removing the preamp and put the filter in its place.
This is where the general rule might not be applicable. It would appear that the preamp being used in this case, might be susceptible to overload, so placing the filter before the preamp might well be best.
In other posts I have commented that the FM/TV emitters are a special case, since they have higher emitting power.
My made-up “rule” is that the ADS-B filters go after the LNA, in all of the normal cases. There is no question.
The very special installations, where an emitter antenna is at meters away from the ADS-B antenna, would require a FM trap in front of them. Those installations are hopefully managed by people that won’t ask around “where is the filter going”.
The RTL-SDR LNA has the FM filter in front. That FM filter probably has less insertion loss in the 1GHz region, than a SAW 1090MHz filter, so I think is a very smart design, conserving the SNR.