Is there any big difference with the comb filter between models? I would assume it's mostly if not completely the same but curious for input from people that have had multiple units as I've been considering getting a second hand unit for film content.

Also I would mostly be looking at older units. HDP, HDQ, XD, XS-3D etc...

Substance has made some very informative posts about this exact thing.

In short the 2124/2144 & old HDQ have the best comb filters. Try searching for his posts (Sorry but I'm not doing it for you )

Found some posts on it already, so thanks for pointing my in the right direction. I would assume pretty much any Lumagen comb filter is better than the stuff in the DVDO units (esp the earlier DVDO units), as honestly I am not very impressed there (though overall the DVDO units are very solid bang for the buck). Though it sounds like the Duo and Edge are pretty good (I have a Duo, and I do like it), I also want to see how Lumagen's no ring scaling looks in person.

I also have the Duo and liked it - definitely agree in terms of 'bang for the buck'. It has the same comb filter as the 2144. As does the Edge though I've not heard good things about the Edge.

The Lumagen scaling is much nicer than the DVDO. The DVDO really overdoes things in the scaling. It 'looks sharp' but has no true extra detail. Adds ringing (in addition to enhancing any ghosting the player has). I found it looked much better by winding the edge enhancement way back into the negative, but that did nothing for the ringing.

Actually I was so accustomed to the over sharpened and enhanced image of the Duo that at first the Lumagen was a bit underwhelming. But now I've been spoilt there's no way I'd go back. Still for the difference in price I think the Duo is a worthwhile option to consider.

I think almost all the problems with the Edge revolve around 240p.

Had this one bookmarked: Suggestions on improving picture quality

Sure there's some others if you go looking though.

Never seen one so can't really comment on it.

My W-VHS decks don't output the menus via component; only composite / S-video. Since the Lumagen's analogue inputs are taken up and this is so not critical viewing they run through the Duo. Turns out they're 240p and the Duo doesn't seem to do a great job with it either. 240p seems to be mostly a retro gaming thing so not important for me.

Well the Duo and the Edge are basically identical. I think the only real difference is the color management the Duo has.

You are right about 240p being a vintage game thing. It's basically a trick. General explanation here if you are curious: https://www.hdretrovision.com/240p/

This website http://retrogaming.hazard-city.de/ specializes in 240p scaling, and has an extensive test archive.
You might want to check it out and decide what you buy after you've read through it. The owner of the site also has an LD player.

Honestly I just want to try out a Lumagen at this point. For the set-up I was talking about in another post I won an auction for an Edge (after having some offers declined on a couple Lumagens) so I'll just be patient an wait for an auction of a Lumagen unit I want.