Sorry for not weighing in earlier, I must have missed the thread.
When the amp is connected to speakers only and nothing else and the hum is constant regardless of the volume control position it is most likely the power supply smoothing caps.
This is highly unlikely in this amplifier and should be discounted in my opinion if we are talking main HV rail capacitors.
It is a dual channel, dual transformer unit with entirely separate supplies for each channel's power stage. The likelihood of both sets of power supply filter caps causing the same issue at the same time is extremely unlikely. People love to blame capacitors and replace them en-masse, only to suffer the same issues after replacement. Golden rule in repair is to fix the problem first, before throwing components at it, shotgun style.
Dual transformer amplifiers (on the same chassis) often have hum issues. Many can be traced to circulating eddy currents in the chassis, poor mounting and electromagnetic interference between the transformers. Poor earthing practices are the biggest issue, along with cable routing and inadequate regulation/decoupling in the VAS stages.
The front end buffering for both channels is derived from a single regulated +/- supply using 78/9xx regulators. The filtering up front is only 470uF per rail, which is at the lower end of what I would expect for a ripple free supply. That said, it may be sufficient.
I would be shutting one channel down completely as a first step. Basically, pull the primary fuse for the
right channel and fire it up. (Should be a rear panel pair marked left and right @7A or so) Report back with your findings. Is the hum still there? You cannot pull the fuse for the left channel as it also runs the front end PSU for both channels.