More confusing stuff to consider, though this has to do with the dvd format itself, rather than dvd-audio: unless standards have changed recently, when a standard dvd player downmixes a 5.1 Dolby Digital track to stereo, it leaves out the LFE channel.
The standard was written that way because it was assumed that the analog stereo output would be directly connected to a TV. And since most TVs only have small, lightly-built speakers and if say an adventure movie's LFE signal* with all its extreme low bass frequencies were allowed to reach them, they would quickly become overloaded and produce very audible distortion (and possibly be damaged).
---> I am not sure if this occurs with a DTS track.
So........if a dvd-audio's Dolby surround mix features bass effects specific to the LFE channel, well, you're not going to hear them.
BUT.....many mixes only use the LFE channel to accentuate bass that is already present in the other channels (the Moody Blues DTS-CDs for example or the DSOTM quadraphonic (
http://www.avsforum.com/avs-vb/showthread.php?t=655946) dvd-audio), so in this case losing that channel is not a problem.
When receivers and other Dolby/DTS processors downmix, that can be a different story. On my own receiver, after some informal testing - listening to and watching the woofer cones - it looked like it kept the LFE in the 2.0 mix (I did this by setting the surround, center and subwoofer channels to "NO" in its speaker set-up menu).
* an extreme example of this: on the DTS track of War Of The Worlds, if I remember correctly HT hobbyists have found it reaches all the way down to 14Hz.