I had XG sound problems for some time with my MkIIXG - non silent - piano. 1999 model and still going strong. I tried a few powered speakers and as was pointed out above, the audio output on the DKC is weak. I solved the problem using a small pre-amp I purchased from a US company via eBay. I fed this from the auxiliary jack on the DKC into the pre-amp and then into my 5.1 main stereo system.
If you want to keep it simple, Yamaha powered speakers may work for you. If not you may need to add a simple pre-amp.