Welcome to the confusing HD club. Yes, D* HD can look good to great. IMHO, OTA looks better.
Unfortunately, SD channels can vary from good to horrid. I've found that on my EDTV plasma, SD can look quite good, it really depends on the compression that D* has on the particular channel. HD displays can also make mediocre SD looks worse, since you have more pixels. That's probably what's bother you. I agree, CRT can look cleaner, sometimes a LOT cleaner w/ SD.
You might check the AVS forums for suggestions on how to possibly calibrate your RCA for better viewing. As your gut tells you, don't believe the rep. SD can look pretty good, just depends on your source, monitor and settings.
BTW, which receiver did you get? Did you get the HD Tivo? Those have gone down to $299 w/ rebates.
