Sony DVP900ES compatability

Ok, I'm Mr.-know-it-all when it comes to audio, but, evidently, mucho stupid when it comes to DVD, etc. Here's situation:

Bought Sony DVP9000ES for Dad as a DVD player only because he's looking for one and I know his set takes Progressive 480 and call him up and, that day, he's gone off and bought a Pioneer! OK, no problem me thinks, the Sony is a great player and I'll just keep it. Hook it up, works fine, but 480 Progressive output scrambled. Chapter two: in Feb of '99 I was in the audio "industry" and got a good deal on a Mitsubishi HD1080 Diamond Hi-def. 50" (model VS 50803)and so took it. I wasn't into video but love sports and movies, and HD basically thrown in, so I went for it. Last three years been running it just with a Dan Wright-modded Pioneer DVD and have been relatively happy (except for the motor noise in the tray when playing, cheap transport, which is one of the reasons keeping the Sony appeals to me). Dealer that Mits relayed through from Mit Co. tells me 1080 model will decode 480 signal from Sony, but nothing on TV menu or backside, and manual it came with is omnibus one and doesn't even go into outputs! I tell dealer this and he says interlace (which Mits will input from Sony) is the same as 480 Progressive and just use that. But me thinks, if thats the case then 1) why does Sony output something called 480 and also something called interlace as if they are different things, which, just from my cursory reading of HT these last few years I remember they are, and 2) if Mits in fact takes 480 then why wouldn't its moniker read "HD1080/480"? Hmmm.

Even though only 3 yrs old, its an early HD set and I'm thinking it doesn't take 480 Progressive only "interlace" and the dealer is just shrugging me off.

I wouldn't use the Sony for audio, only a DVD player. Before I call Mits, I thought I'd ask you guys. So, am I about to place a new Sony up for sale?

Thanks for the help!