I have to tell you that when someone says its quiet, it is subjective and IMO PA has to really change their noise level scale to be more accurate because they think some loud guns are quiet while some quiet guns are loud. I think a Beeman P17 is loud. Barely offensive IMO to ME.
I would rate a 177 Benjamin Marauder on the quietest scale when it comes to off the shelf factory quieted production guns but you can get quieter than that if you have a different gun with a very good LDC. You should just try it out first to see if you can live with the noise, who knows? You may not think it is loud and your level of quiet may not be as critical or needed as others need it to be depends if your neighbors are gun activists or not, really it does. If you see TRUMP signs around your neighbors' property then I wouldn't really worry about the level of loudness.
I have never owned a Flash nor Flash Pup so I can't make an accurate assessment on that but based on the AT44s in which their air cylinders unscrew, (I don't know if the Flashpup unscrews to do this mod) one can plug a hole to lower the power. The internal hammer spring adjustment is not enough to significantly reduce the power level on those guns. Your only other choice is to shoot heavy pellets.
I would get in contact with someone like DonnyFL with your quiet questions. I prefer to buy my guns without any factory LDCs because I found that aftermarket ones seem to work better in regards to effectively quieting the blast.
CA