I have come up with an additional Mod. for the factory McIntosh amps also.
According to this site
http://jeremy.geek.nz/2010/06/17/mcintosh-ef-1080i-information/, and my own observations, the amps are based on ST TDA729* series amplifiers. These can do up to 60W music power (for the TDA7296). However this does depend on the voltage available to the amplifier.
McIntosh has set the internal supply rails to +/-26V. This is on the safe side of the amplifiers, and as the maximum is +/-35V (TDA7296).
I have increased the voltage to the amplifiers, to 29.5V. This allows more 'Swing' on the output before clipping, and allows additional power output.
The mod is very simple. A 50k ohm resistor
http://www.altronics.com.au/p/r7599-51k-0.25w-metal-film-resistor-pk-10/ between pins 1&7 of IC601 as found in this service manual:
https://nzlamb.files.wordpress.com/2010/06/ef-1080i-c.pdfI used 2 100k resistors in parallel. A 47K resistor will be ~30V, 51K ~ 29V
I don't think the power supply will be able to do much more than +/-30V with a battery voltage below 13V. The OEM configuration could not maintain +/-26V below 12.5V input.
Bench testing on PSU duty cycle doesn't change much from the original.
The increased voltage available to the amplifiers will mean that they dissipate more power. Power= Heat=Bad. So if you want to run the amplifier with this mod continuously, buy a bigger after market one. However if your music has a large dynamic range, and you were clipping in the louder sections, this might mean you will not risk destroying the speakers due to clipping.