McIntosh- Sweetest, Smoothest sounding Solid State ?

Who cares?  
Transparent is the goal.

It is, but if the amp is not smooth that is an indication of distortion. Its hardly being transparent at that point. 

This is only slightly off topic, but what sonic coloration does an autotransformer impose on a McIntosh power amplifier?

My understanding is that its there to reduce coloration since the amp makes less distortion because the autoformer loads the amp at a higher impedance than that of the speaker. All amps make greater distortion into lower impedances; conversely less distortion into higher impedances. This is a simple albeit expensive way of making the amp sound smoother and more relaxed and its doing it by reducing distortion.

Its relaxed character in the highs isn't a frequency response error- its that way because of less audible higher ordered harmonics. The ear uses the higher orders to sense sound pressure and so is keenly sensitive to them; it also assigns a tonality to all forms of distortion and the higher orders are assigned 'bright' and 'harsh'. So reducing them results in smoother sound.

“i currently have a parasound a21+ driven by a cary audio SLP 05 tube preamp.  
i liked the parasound at first but after 7 months or so it is sounding slightly sibilant and less refined.”

I’ve had a Parasound A23+ for well over a year now and it’s not sibilant at all and I do find it refined in my professionally calibrated system. I tried a high end Class D Gan amp recently, and the Parasound was light years better in every way. Perhaps something else in your chain isn’t up to par, misadjusted or maybe room acoustics are bad.

I do love mcintosh for there looks and build quality.  They are also very serviceable. 

I have a old Mc2300 just sitting in my rack for fun.

Also have a Mc452 which sounds great and use it to power a 15 inch subwoofer. 

However I use a JC5 to power the main speakers. 

Enough said.