I have a CD-DSP, and love it; I haven't been able to try any other DAC's with it, but there are a couple of little tweaks that I'm trying.
I tried using an MSB Digital Director for it's jitter reduction function in between the Digital In & Out, but unfortunately, the Krell is set up so that if you select an Input other than CD (to receive the less jittery signal), it cuts out the CD output! I'm looking at possibly installing an aftermarket clock instead (LCAudio LClock3, or Audiocom Superclock 2, or possibly the Elso Kwak DIY module).
Another one is al la Eichmann 'Toppers'; I got some small cheap surplus heatsinks (reasonably heavy) and put them on the top of the unit, one on the cover & one on each side, to damp out cabinet resonances (take care not to block the vents). Tap the cabinet gently to hear where it's 'buzzy', and place the heatsinks there. It's not a bad idea to put a layer of masking tape on their bases so they don't scratch your Krell!
I did this for all my gear, even on top of the speakers, and I'm pretty sure I can hear an improvement in the soundstaging.
The last is to flood the underside of the CD with Blue (or even UV) light from LED's; apparently this make a difference too...
OK, so this was more of a DIY approach than sticking an aftermarket DAC on, but who isn't interested in better sound for less $$? : )
Have said all of that, I too would like to hear if anyone has tried a DAC that improved the sound of the Krell...