Aren't all channels above 600 supposed to be HD only, including VOD films at 751 and 752?
Yes and no. Those channels always broadcast in HD mode (1920x1080i or 1280x720P) but the source is not always in HD quality. Producing a TV show in HD is quite expensive, so, some of them are now produce in HD (SRCHD is doing a nice job on his production) but don't expect everything to be in HD. There is more, years after years, but a lot is still up-converted stuff (and the up-conversion quality is quite different from a HD channel to another).
For channel 751 and 751, it's always in HD. There is no up-converted stuff there but don't expect the same quality as a DVD-HD or Blue-Ray. First, those HD channels are encoded in Mpeg2, with a bitrate around 14.3 Mbps and HD disc is around 20-25 Mbps in mpeg4 format. This is a very big difference and the studio are not stupid. They want that difference to exist. So, the studio doesn't always send their best HD copy. Sometime, a movie in 2.35 could be crop to fit in 1.77 format and sometime, the DD5.1 sound is not always there (I never understand that one...).
The bitrate for those channel may seem low but in fact, it's not really the case because the movie is encoded in advance (like on DVD), not in real time (like OTA station). So, the quality is good enough, if the source was in good quality also, which is not always the case. 2 years ago, those channel were still broadcast around 11 Mbps. This bitrate was not really good. The BEV version was even worst because they re-encoded the source in 1280x720P, which was in 1920x1080i. I think they are still doing it but the bitrate is now around 14 Mbps also (fitting 2 HD channels by TP).
For Videotron VOD-HD, there is no up-conversion also and I don't know about Cheech but I analyzed the broadcast for other HD show (Insectia, Maurice Richard, Un dimanche à Kigali) and the bitrate was always around 12 Mbps (fitting three HD by qam) and the resolution was, in fact, 1440x1080i. But, since it's also encoded in advance, the quality is not so bad. Yes, it could have been really better if those show were encoded in 1920x1080i, in full bitrate (around 18 Mbps) but, we do need the TV to see the difference.
About TV, a model like yours (a lot like mine) is considered to have a resolution about 800 x 540P. No, it's not 1080i. The signal in 1080i is only accepted. So, this is still very close to a DVD resolution. It's quite normal that you don't see a big difference with full HD stuff, but, believe me, the difference is quite obvious on a Full HD TV. The difference between a real good DVD and badly encoded HD channel is really not that big but the difference between a good DVD and a good HD channel is really there. It's even bigger when compared to a DVD-HD or BD-DVD in 1920x1080P, encoded in Mpeg-4 with a good bitrate.