I've never used the ART Dbs or the Dbx preamp, so I can't be of much help. The real question is whether or not these units are the same, better, or just different compared to the preamps/converters in your Korg. Alot depends on the quality of the Korg D16. I assume, given the price, it's a decent stand-alone recorder perhaps in the same ballpark as the 16 track Yamahas. But I've never seen a compaison of these types of machines against different classes of preamps/converters. The 3d Audio site has a number of comparison CD's looking at differences in preamps and converters, but they don't include any standalone multitrack hardisk recorders. Personally, I would like to see some type of "shootout" comparing recorders by Fostex, Tascam, Zoom, Korg, Akai, Yamaha, etc., but that's another thread!
I guess the only way to tell would be an A/B comparison using your Korg and and an external preamp/converter. You could purchase one from a dealer with a fair return policy or maybe rent one if you live in a metro area with a rental shop.
I also had another thought. Have you tried using your Korg in the 24bit versus 16bit mode? From what I read, these machines can record 16 tracks at 16bit or eight at 24 bit. Although eight tracks can be limiting, you might see whether or not the 24 bit mode makes a difference. The 16/24bit issue also makes me wonder about an external converter and how it would work with your setup. If you had a 24bit external converter and you were recording 16 tracks at 16bit, how would the Korg (or the external device) handle the conversion. Would it use some type of dither or would it truncate or throw out the additional eight bits? I don't know if these are issues, but they are worth thinking about.