Thanks for the very helpful responses. So I may have to put the antenna on the roof after all, though from what I've read I thought the lower the frequency the less likely it is to be attenuated by structures such as wood and roof shingles and being receivable beyond the horizon. However I'm now learning that higher noise susceptibility is the Achilles heel of VHF Low band reception (and digital reception is nearly all about S/N right?) and that the advantage of roof mounting may have just as much (if not more) to do with furthering the antenna from the noise sources as it is about signal gain at VHF Low band frequencies. Possibly a high VSWR is also an issue as per this nice article
http://www.tvtechnology.com/conferen...visited/183659, as all my electrical wiring is in metal conduit running across the floor of my attic 2 feet under my antenna. NE Illinois code specifies all electrical wiring must be in EMT conduit which may help however with power line noise emission, while causing possible high SWR due to its proximity. Thankfully the weak channel 6 analog station from the same TX site can sort of be used as my poor man's test equipment for seeing noise on the signal. I'm still wondering why my wireless n router causes noise in the VHF low band.
I may hold off putting it on the roof until the 4.8 dB increase, because my 60" Sony EX500 looks like it on the verge of locking on, as it does show a "Now tuning" for about 25 seconds when I enter 4.1. On empty channels it show "No Signal" right away. Unfortunately, the signal diagnostics don't display anything at all unless there is at least a 15 dB S/N from the signal. My older Sony digital XBR960 CRT at least shows a signal level even when not locked in.