...with huge transformers. I wonder if that's the difference?
@dhcod Yes, sort of. After dealing with the large potted transformers on the RM-9 which accounted for quite a bit of it's weight, Roger wanted a tube amplifier with a small footprint that he could easily carry around. So for the RM-10 he made much smaller transformers and compensated for them with networks in the circuit. The amplifier is extremely flat from 30 Hz to 20k Hz, but things start easing up below 30 Hz, so that might be what you are hearing.
At the time of it's design 35 watts from a pair of EL-84s per channel was double what other designers at that time were getting out of two pairs of the tube. Putting 700 volts on the plates was unheard of, until Roger did it. This is why the amp punches well above it's weight and has a pretty nice set of balls.