My experience with cable boxes and DVI/HDMI output matches yours - the issue has less to do with the TV's inputs and more to do with the cable box. Since you have DVI input (and it presumably supports HDCP, based on the fact that the cable box worked with it and I don't know of any DVI cable boxes that lacked HDCP), I'd consider using it for the DVD input.

The BD50 looks to be a promising player, but it will not be cheap and the DVD upscaling (based on what I've seen with my BD30 using the same video scaling/deinterlacing chipset) isn't going to be stellar. It'd probably be close to what you were seeing with the RP56, and of course Blu-ray discs are very impressive looking, so it's not a bad option to consider if you are thinking about starting into Blu-ray.

The OPPO 983H and 980H can both to 480p over component with copy-protected discs and can scale to HD resolutions with non-copy-protected discs, although the 983H doesn't use its expensive ABT chips with the component output - the original 971H was the only OPPO that couldn't do above 480i with its component output. My OPPO 98x player comparison might be of interest to you - I tried to summarize my experience with all three current OPPO players, since I was part of the beta test team for each of them. As my comparison says, none are bad choices. The 983H is certainly their best player, but it comes with a hefty price tag (for a DVD player) and is backordered right now - not a great fit for someone with a dead player in their rack. If you liked what Panasonic did with the Faroudja chip on the RP56, you will likely be amazed at what the 981HD can do (prior to getting my 971H, I had a Yamaha with the Faroudja chip in it, and the 971H was clearly a better player). The 981HD also lacks a component output entirely, so we'd need to be certain that you have a DVI-HDCP input on your Toshiba.