Yup, all DC voltage checks are referenced to ground, except perhaps the method to measure bias. It's probably measured across a small value resistor instead of from the high side of that resistor to ground.
The larger hum in amp #2 may be due to more apparent mismatching of output tubes. This amp doesn't appear to have a way to adjust balance of a single pair independently, and apparently only has the means of measuring the aggregate quiescent current of the entire stage.
The amp should be dead quiet at idle, when connected to a test speaker, even with your ear up within a few inches of the cone. On an oscilloscope, if you connect the amp to a dummy resistor load (say 8 ohm load on 8 ohm tap), and probe the signal across that load with the oscilliscope when no music is playing and with the input shorted, you will probably read a 60 Hz sine type wave at less than 2 mV peak. That value or less is approximately what it should read if there are no hum issues with the amp.
If there is a ground issue in the amp's internal wiring, or any other sort of hum issue, it will show up quite obviously on the scope trace. Hum will be probably either 120 Hz or 60 Hz. You can determine which it is by finding the period of the hum signal (how many miliseconds from crest to repeating crest) and taking the reciprocal of that. If you have 120 Hz hum, it would be indicative of power supply switching coupling into the output stage or into the audio circuit. If you have 60 Hz hum, it would be indicative of mains voltage coupling to the grid of the voltage gain stage, such as through a long or unshielded wire from RCA input through to the grid of the first amplfiication stage tube. A bad ground (sloppy solder joint) or less than stellar ground system wire dress could also be the cause of hum. First place I'd look though is at the potential mismatch of output tubes. You might try swapping tubes from the good unit that is "hum free" into the other unit and see if that changes anything. That will tell you if it's the tubes or the amp.