Dave -- This is completely normal what you are observing. The ST-35 reaches its full power output rating with a 1.0 vac signal applied to each of its inputs. But since you are using the Stereo Inputs of the MC240, that amplifier then requires an input level of 2.20 vac at those inputs to reach its rated output of 40 watts in each channel. So if you are driving each channel of the ST-35 with 1.0 volt to reach its rated power output, and the same signal level is also being applied to the stereo inputs of the MC240, that signal level will cause it to only develop 8.27 watts per channel. So the Dynaco sounds louder.
In this scenario then, the difference you are hearing is directly related to a GAIN difference between the amplifiers. Based on their respective rated power output levels, and their input level sensitivity to produce that power output, the ST-35 has a gain of 16.7 relative to the 16Ω tap, while the Mac (when using the stereo inputs) has a gain of 11.5 relative to the 16Ω tap. So all else being equal then, for the same signal level applied to these to amplifiers, the Dynaco will sound louder -- but not always.
Just because the Dynaco sounds louder in this scenario doesn't mean it has more power, or that the noted sound level difference will remain consistent at all volume levels. The POWER rating of an amplifier is the ultimate determiner as to how loud a system can cleanly play (all else being equal), given the necessary signal level being applied to an given amplifier's input -- which can vary wildly between and within various models. Some amplifiers only require an input level of 0.5 vac or less to achieve rated power output, while others, much more as shown above. Specifically to your scenario then as you have it connected:
While at a normal listening level the Dynaco plays louder (because of its greater gain as explained), it can do so because at this level, both amplifiers are no doubt operating well within their rated power abilities. However, if you continue to turn up the volume, at some point, the Dynaco will reach it's full power output capability, where any further rotation of the volume knob will simply cause it to start distorting due to overload. Up to that point, the Dynaco will always play louder than the Mac because it has more gain. However, if you then continue to turn up the volume from that point, the Dynaco will simply become more and more distorted but get no louder, because it has now run out of power. The Mac on the other hand will continue to get cleanly louder -- first to the point where it then equals the maximum sound level that the Dynaco could play cleanly at (remember it was not playing as loudly before continuing to turn up the volume), but then when continuing to turn the volume up even further, the Mac will play cleanly louder than the Dynaco ever could -- because it has more POWER output capability.
When comparing power amplifiers, gain and power are closely related, but it's often a confusing topic to many (and therefore erroneously analyzed) if it is looked at simply from the standpoint of how loudly one amplifier plays versus another when looked at from the standpoint of a given volume control setting to produce a given sound level. While they are closely related then, they are also distinctly different: Gain determines the input signal level required of an amplifier to reach its rated power output, while rated power output then determines how loudly the system can cleanly play.
If you want more gain from your Mac, then use the Twin inputs. With these inputs, then besides having greater power output, the Mac will also display more gain than the Dynaco, requiring you to actually back the Twin input gain knobs down from a full up setting to make the two amplifiers play at similar sound levels.
I hope this helps!
Dave
