Can you bring your main system's integrated amp and try that in place of the Decco? If so, do that without changing the speaker positioning. If it sounds equally crap, you know it's not the Decco.
If the integrated fixes the problems, it may be you don't like the DAC section of the Decco or it's not driving the Dyns well enough.
If the integrated fixes problems, try using the Decco as a preamp/DAC with the integrated as a power amp.
Experimentation should tell you what you need to know. All it costs is a bit of time and effort. I don't have a Decco, so I'm not defending it nor am I blaming it.
If the integrated fixes the problems, it may be you don't like the DAC section of the Decco or it's not driving the Dyns well enough.
If the integrated fixes problems, try using the Decco as a preamp/DAC with the integrated as a power amp.
Experimentation should tell you what you need to know. All it costs is a bit of time and effort. I don't have a Decco, so I'm not defending it nor am I blaming it.