The only reason not to is cost. The reason to is mains isolation. The distance between them is irrelevant provided a shielded cable of suitable gauge is provided. Yes shielded as DC is easily contaminated.
If you don't have separates, and you're brave enough, open the case and with a sheet of aluminium, cut and fit shielding around the power transformer and if possible around the DC power board. Don't stop there, do all your equipment and you'll be shocked at the improvement, no matter what the purchase price.
