The Best DC supply is a car or motorcycle battery. It is completely
noise-free, has enough power to eliminate audio clipping, and can
supply many hours of listening.
The drawbacks are the need to be recharged and the need to keep it
well clear of electronic equipment because of corrosion caused by ions.
Of course, you cannot use it with the charger when listening as most
battery chargers are quite noisy, but I have used a motorcycle battery
with a 2 amp charger that was not too bad (Schumaker Mity Mite).
To repeat, do not keep a lead-acid battery near (within 10 feet) of
any electronic equipment: it will eat hell out of the boards. But it
will give you the best DC supply and it need not cost too much, either.