I heard once that the Mac receivers were designed by one of the "kids" (maybe Franks son), not the regular engineering department.
We once were able to go through a pile of McIntosh Clinic results while they were setting up for the evening. The Dynaco 120 would meet McIntosh specs down to 30 Hz, then shot off the graph because it ran out of power supply. I had an idea for maybe inverting phase of one channel, then inverting it back to normal at the speaker connection. This way, low frequencies would be mostly out of phase and both channels would not be hitting the power supply simultaneously.
Heathkit AR-15 would meet McIntosh spec up to near 20 kHz, where distortion would rise to 1%. Of course, these tests were only Harmonic distortion.
As for the tube equipment, McIntosh had transformer patents and made their own transformers, giving them an edge. Looking at schematics for the early solid state equipment, the design was rather basic. The main points were the output autoformer (full power at any impedance and no possibility of DC on the speaker if the output shorted) and extreme build quality. They could also deliver full power continuously, as in a non musical industrial application. Fisher and many other units were designed for music reproduction where average power would be 10 dB below maximum to prevent clipping. Transformers and heat sinks would not heat up as they would under continuous full power.
I never trusted Marantz after a review of the 10B tuner. Either Audio or High Fidelity magazine did a test and it was quite mediocre. A few months later, they had "found an error" in the test setup, did a retest, and everything was wonderful. I'm guessing that the "error" involved cash, and the original test report was probably the correct one. More recently it has been mentioned that some of the 10B specs were inconsistent and would have been impossible, especially in that era.
An engineer at Collins Radio in the 1960s mentioned that he thought that Scott tuners were "hotter" than Fisher. Today, with the decline in program and technical quality, combined with being 40 miles out, rather than 10, and the FCC not really doing anything about crap stations on the same channel in the opposite direction, I would not bother paying any kind of premium for an "excellent" tuner. Just not worth it.