Oppo 983 is the next step up in price/performance. After Oppo, not too many units I know that play DVD and have variable volume control. The Cary DVD-6 does, but has fewer volume step in it's standard build. I think Cary offered an upgrade which provided more volume steps for greater control of output volume. But the Cary is out of your price range.
I use an Oppo 970H which was mod'd by EVS to their level 2 status. It is very good, competing with dedicated CD players which retail for up to $2K-$3k IMHO (yes, I've owned dedicated CD players in that price range).
If you're interested in mod'd units, you could check with Ric Schultz at EVS for pricing. His web site is www.tweakaudio.com.
Another possibility is adding an external DAC which has variable output. A couple of manufacturers of DACs with variable volume are Birdland and Monarchy. I think they are still above $300 used.
Enjoy,
TIC