I just set up my sister-in-law's Samsung SIR-T451 QAM STB (HD tuner). Interesting that the outer box specifies QAM but the owner's manual says ATSC/8VSB only, no cable support.
The bad news: We got several HD locals okay, after adding 10 db of gain right out of the wall (offset partially by about 7 db in a 1:4 splitter). Before that most were pixelating heavily. So she gets KOMO-DT, KING-DT, KONG-DT, KCTS-DT (all 4 sub channels), KCPQ-DT, KTWB-DT all fine. KIRO-DT barely comes in, flashing on, pixelating, flashing off, and so on. Tried attenuating, to no avail, just goes away completely.
The good news?: I saw digital versions of many of the Comcast analog channels, including main locals, and other known analog cable channels. About 30 or so channels in all. BUT, they appeared heavily compressed and had motion and heavy picture change artifacts, looking much like an SA TiVo recording at Basic quality. I would say they have some adjusting to do to bring these up to snuff. Compared to SD local channels I receive from DirecTV, the Comcast digitized locals were pretty bad. Here's hoping they will continue to tweak and improve them. I left the list of channels numbers (for the digitized analogs) behind, but I think they were mostly in the low 70's, with ten subchannels for each main channel number.
I guess we get to call Comcast out to see if they can get a usable signal delivered to the condo.