Hi brownbear,
I'll take a shot at it. Each channel of the MC 452 has two independent 225 watt power amps. These amps and all their input circuits mirror each other. They are fed a balanced input signal with one amp inverted with respect to the other. The outputs of these two amplifiers are then summed in the output transformer to get to the 450 watt output power. The advantage of all this is any distortion and noise that is common to these individual amps is cancelled out while the balanced signal appears at the output.
The MC 312 is a normal complementary symmetry design, where a single power amplifier drives the autoformer.
The disadvantage of the Quad Balanced design is the parts count is doubled and the outputs are not referenced to ground so can not be used with some speakers setups that are bi-amped.
Lots of McIntosh amps are quad balanced; MC 2KW, MC 1201, MC 501, MC 601, MC 611, MC 301, MC 602, MC 402, MC 352, MC 452, MC 462, MC 352, and MC 2301.
Apparently they like that design!  The 602, 402, and 352 were released before marketing invented the Quad Balanced nomenclature.
As far as the sonic differences, I have no idea. I do think the non-quad balanced designs are offered to hit specific price points in their lineup.
Tom
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Main System:
Amati Futura Mains
Amati Homage VOX Center,
Proac Response 1sc Rears,
Three MC2301's for L,C,R
MC 602 for the rears
C 1100, MX 151, MCD 1100, MR 80
Nottingham Dais with Wave Mechanic
Sumiko Palo Santos Presentation
SurfacePro 3, RPi 4, ROON, WW Starlight Platinum USB, Schiit Yggdrasil, Benchmark DAC3 HGC
MX 151, OppO BDP-95, JVC RS-500 DILA projector, 106" diagonal Stewart Luxus Screenwall Deluxe with Studiotek 130 G3 material.
Lake House:
Ohm F, MC 275V, C2300, MR 77, Rega P3
OnDeck:
McIntosh MAC 4300v
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