Your question is not stupid by any means. Only in the case where you are very close to having stable and reliable reception would 1 or 2 dB make a noticeable difference.
Your TV Fool report is a guide, an estimate based on sound theory and the math behind that theory, but it is only an estimate due to the complexities of the real environment. As the terrain becomes complex, the accuracy of the estimate is reduced.
http://www.tvfool.com/index.php?opti...57#how_to_read
If your TVFR is 100% correct, then simply look at the predicted noise margin of a particular signal, try to find an antenna who's gain is sufficient to add to the NM figure so that the sum is a positive number. Amplifiers produce noise which will subtract from the net NM. The gain of an amplifier will not add to the system NM, ONLY antenna gain is able to add to the net NM of your system. Your real world results strongly suggest that the predictions shown in your TVFR are not 100% 'on-the-money' - That's to be expected given the complexities of your location.
As a rule of thumb, reliable reception will require a net NM at least +10 dB. even then signal level can fade and interfering signals can come and go. Higher net NM is desirable up to the point that it would cause overloading of an amplifier or tuner.
The DB-8 is a UHF design which can serve as a VHF antenna in some situations. The ability to receive out-of-band signals is true of virtually all antennas to some degree. Yes you can use the DB-8 with an antenna designed for VHF reception. If you combine the signal with a
UVSJ, the filtering action of the UVSJ will block most of the VHF signals from the DB-8 and most of the UHF signals from the VHF antenna.