IMO it depends on your monitors, how good your ears are, and how much $$$ you got laying around you want to spend.
If you have a really nice pair of monitors and have good ears (ie have been trying to mix & produce for over 5 years) you will probably hear a difference with a better converter than the Delta. What this means for a D/A converter is that you make better decisions when mixing or you have good conversion if you are sending your signal out of your DAW, then back in (so a really good A/D helps there too).
Also, when talking about converters dynamic range is only one piece of the picture. What it also depends on is how much jitter is introduced in the conversion, which is a function of the clock. If the clock is bad it can start to make the dynamic range meaningless.
I'd say the converters you have now are fine, unless you are at the point in your music you want to spend $1000 for just 2 channels of D/A conversion . . .