It's not clear what Apex model you have, but if you have the $200 model 600, or similar, that resolution limit of 500 seems reasonable. Clearly something has to give at very low hardware prices.
Using the Avia DVD test disc, I can clearly distinguish resolution lines up to ~625 with my DVD player. (Don't hold me to that number, it's been several months since I ran the test.) It's a 16-month-old Toshiba SD-6200, which was a mid-price level unit when I bought it (~$700), but it's much less expensive now. While DVDs may have a theoretical resolution of up to 720 pixels horizontally (both 4:3 and 16:9), prefiltering of signals, which depends on the disc producer, typically trims this resolution. Agree with the comments above. I've encountered a few high-quality DVDs that seem to approximate the appearance of some low-quality HDTV programs. That is, you can crank down the production fidelity of HDTV so that it appears similar to some DVDs. -- John