It sounds like you have a DirecTV HD DVR with TiVo (originally made by Hughes), along with a Pioneer 510H DVD recorder.
I have a similar setup, except that I have the Pioneer 520H instead of the 510H. But they both behave the same with respect to widescreen recording.
Here is how it works for me, which I believe is the same as your description.
I originally record an HD 16x9 program on the HD TiVo. Then, to make a DVD copy of the program (in 480i of course), I set the HD TiVo for 480i resolution and Full aspect ratio (instead of panel). Viewing this on a 16x9 TV gives a full-screen, widescreen picture at 480i. Viewing the same source via s-video on a 4x3 TV results in a horizontally squished (or vertically stretched) picture filling the 4x3 screen. This mode gives you the best available picture detail from the HD TiVo on the s-video output. I believe you described the same setup on your HD TiVo.
Unfortunately, the HD TiVo does not provide the widescreen flag along with this anamorphic output. Recording this on my Pioneer DVD recorder results in no widescreen flag on the recording, even though the Pioneer would capture and record it if it were available. Playing this back on a widescreen TV is no problem because the original horizontally squished 4x3 picture is now horizontally stretched to fill the 16x9 screen. All is well so far. As you likely already know, this is exactly how a commercial DVD provides an "enhanced for widescreen" picture.
A problem occurs when watching the same recorded DVD on a 4x3 TV. Now you see the horizontally squished 4x3 picture. If there had been a widescreen flag present on the DVD, the DVD player would "letterbox" the video by adding black bars above and below the picture to give you the proper 16x9 video. Or, the DVD player might crop the sides of the picture (a.k.a., pan and scan) and display a full-screen 4x3 picture, minus the left and right edges that were cropped in order to fill the screen. The 4x3 TV display options you set in your DVD player determine which of these display modes applies.
So, to set the widescreen flag that will enable DVD players to display the video properly on a 4x3 TV, we import a copy of the original DVD produced on the Pioneer recorder into our PC or Mac, and use the appropriate programs to force the widescreen flag to be set properly. Then we burn the resulting files onto a new DVD "master" with the properly set widescreen flag.
You are getting the maximum available picture detail on your recorded DVDs using this process. The only way to improve the picture quality would be to obtain a newer DVD recorder that might have better analog to digital conversion and/or better MPEG-2 encoding.
A simpler process with fewer steps would be possible if you had either:
1) A source of 16x9 programs that actually sent the widescreen flag at the time of playback (no such device exists for DirecTV HD DVR service);
2) A DVD recorder that lets you manually force the widescreen flag while recording from the HD source playing back at 480i. Some Sony recorders and a few others provide this capability.
I looked into the cost of buying a Sony with HDD and decided it cost too much for me. So I continue with my current process. I am quite pleased with the results, which video-wise rival commercial DVDs, IMHO, when viewed on my HDTV.
I wish we had a way to preserve the DD 5.1 audio through this process.