I highly recommend the Bel Canto DAC 1.1, at around $550 used. I owned the earlier CI DAC (your price seems to indicate the new one), and while it was pretty good, I found the Bel Canto clearly superior in all respects.
Other small form-factor DACs I have also owned:
Theta Cobalt 307: nothing special, dated
Musical Fidelity X-ACT DAC: un-musical and dull, dated, I hardly think it would improve even the oldest player
Musical Fidelity X-DAC v3: dry (perhaps "brittle") but quite detailed (like the Classe DAC-1 I briefly owned), but posessed of an impossibly annoying input-scanning "feature" that could not be disabled
I haven't tried the others on your list, but of the whole lot, if I were to do it again, I'd stick to Bel Canto or CI, and if the Creek came up cheap, I'd consider giving it a try. I know very little about the Ack!, but it might be battery-only.
Other small form-factor DACs I have also owned:
Theta Cobalt 307: nothing special, dated
Musical Fidelity X-ACT DAC: un-musical and dull, dated, I hardly think it would improve even the oldest player
Musical Fidelity X-DAC v3: dry (perhaps "brittle") but quite detailed (like the Classe DAC-1 I briefly owned), but posessed of an impossibly annoying input-scanning "feature" that could not be disabled
I haven't tried the others on your list, but of the whole lot, if I were to do it again, I'd stick to Bel Canto or CI, and if the Creek came up cheap, I'd consider giving it a try. I know very little about the Ack!, but it might be battery-only.