Just in reference to the Denecke AD-20, if it really has full 20-bit performance then it is about as good as you're going to get in this class of equipment, even if 24-bit is your preferred recording medium. 24-bit PCM is a terrific, super-wide-dynamic-range data format but no A/D converters IN THE WORLD are capable of 24-bit performance and the noise and level settings prior to the recorder are rarely optimal in live recording, so a considerable amount of the medium's potential always goes unused. The very best, multi-multi-thousand-dollar rack-mount studio units are only ca. 21 bits under optimal conditions, which is to say, in a laboratory setting where the incoming analog signals are as nearly noise-free and as predictable and controllable as it is possible to make them. In other words, even that 21-bit level of performance depends on everything which you and I don't have at our live recordings.
Main thing is, once you get even two bits beyond 16, you have enough dynamic range "cushion" to make a good 16-bit transfer every time, even if your levels were set conservatively at the concert; or you can push the recording levels up to nearly 0 and get a recording that is so quiet, its noise floor is well below the noise floor of your listening environment at all frequencies; or you can split the difference to your heart's content.
But believe me, if all venue/microphone/preamp/converter combinations had the dynamic range equivalent of 20 bits, things would be remarkably better than they generally are today. 20 bits is nothing to sneeze at, and the "24-bit" equipment at commodity prices is often not much better than honest 16-bit equipment in its real-world dynamic range performance, especially given the quality of analog signals which are the best we can feed in under the given conditions, and the limitations of a (frankly) cheap recorder's analog circuitry.
At some point, when one part of your setup is already 100 times better than the rest of your setup, you have to admit that raising that figure to 400 or 800 isn't going to help things any--that if you want to make significant improvement, you need to improve the thing which is the weakest link in the chain, not the strongest.
--best regards