I just rechecked my original tests without a preamplifier used pointing to Chicago and South Bend.Whoa! I did not say NMs in the 20's would be an overload concern. I said NM's in the mid to upper 40's plus a 10 dB gain antenna plus a 25 dB gain preamp plus many stations in the UHF band MIGHT be a borderline overload concern. On top of all that I know the strong signal characteristics of the RCA and LNA-200 preamps aren't the best.
TV Fool might predict a NM of +25 dB but that goes up to +35 dB with a 10 dB gain antenna. System losses might take that back down to the low 30's. Take a look at my Noise Margin graphic for TV Fool to get an idea of where you signals are. They're not bad. TV Fool shows all my local stations in the range of -10 dB to +10 dB. Yet when all is said and done I measure +25 dB to +45 dB.
I don't know what 3 out of 5 bars means. I assume it's a Signal Quality meter no matter what they call it. WSBT and WNDU are both strong signals. If the Signal Quality is low then it's due to multipath issues. I don't think you've said anything about how clear your view to the transmitters is. Trees or buildings in the way? No antenna or preamp will fix that.
Unfortunately, my new TCL Roku based TV does not have (that I am aware of) any signal strength meter, so it shows only the strength from one to five bars, similar to many cell phones.
I do have some trees about 50 feet from the antenna, but fortunately there is a good size gap to aim through in the proper direction (only affecting Chicago - the sight line towards South Bend is reasonably unobstructed).
As it turns out, I did get 4 out of 5 on several stations pointing toward South Bend, although Channel 28 was a disappointing 2 out of 5. Similar for Chicago, where I did receive 4 out of 5 on the more notable higher power stations.
Seeing this, I can see the potential for an overload situation with a preamp, especially if two antennas are connected and they add signals instead of subtract signals for certain stations.
I guess all I can do is try it and see what happens. I know what stations SHOULD be showing up (everything that showed up with a single antenna without a preamp), so I'll quickly know if overload is an issue!
Again, thanks for the information.