I own several DVD-A recordings. One of my players is equipped for DVD-A audio, but I tend to prefer to play the discs in Dolby digital 5.1 or DTS if it is offered. However, I only play them in 5.1 sound. I cannot say whether or not you would get a great improvement if you played a DVD-A on a regular DVD player in only 2.0 sound. If I had to hazard a guess, I'd say no.
I imagine it would be like listening to a Dolby Digital 5.1 movie in 2.0 sound. When I first bought a DVD player, I only had a Dolby Pro-Logic receiver. I had 5 speakers and a subwoofer, but the receiver was only capable of 2.0 surround sound. I thought that sounded pretty good until I upgraded to a receiver that could decode Dolby Digital 5.1 and DTS. After the upgrade, it was as if I had a new DVD collection. Not only were all the channels more discrete and the surround effects more aggressive, but the over-all sound had more presence.
On several threads, I have written favorably of the sound on the DVD-A Mahler 2nd by Zubin Mehta. With 5.1 sound, all the separate parts that make up the finale come though loud AND CLEAR. The organ in that recording is the best ever, IMO. Upgrading to 5.1 sound is a good thing for a Mahlerphile. You will be able to listen to DVD-As in Dolby Digital. And some DVD videos contain stunning sound as well. The Mahler DVD videos by Haitink and Abbado all feature both Dolby Digital and DTS, and sound great, IMO (though neither of their 2nds has quite the organ of the Mehta DVD audio).
It is too bad there are not more DVD-As made. I guess SACD appealed to more people because the discs are hybrids, and you can play the normal CD track even witout the SACD player. However, I would recommend upgrading to decent 5.1 sound before jumping into a format like DVD-A or SACD. My sound system is not expensive, but most people who hear something in 5.1 on it are usually impressed.
BTW, another great-sounding DVD-A is Mehta's Carmina Burana. I first played it on my DVD-A capable player, and was quite unimpressed. The recording had a sort of muffled sound (which was surprising, as the Mahler 2nd had sounded good in DVD-A or Dolby Digital).l I was going to sell it for a couple of bucks at Half Price Books. Then, I played it in my best DVD video player in Dolby Digital, and it really kicked butt. However, when you select that option on certain DVD-A discs, you lose some special features. On the Mahler 2nd and Carmina Burana recordings by Mehta, I could no longer access the texts to the music. But, I could live with that.