Have read this thread with interest.
Ordered the 976 yesterday, will replace my 990 I have owned since 2006.
I have a HT cabinet which house all my HT gear on both ends of cabinet.It is inclosed
I have oblong cutouts under unit ( towards front) and 5 inch cooling fans on back panel towards the top, these fans activate at 100 deg.
I have yet to have any problems with any piece of equipment failing.
That includes 2 amps with out cooling fans.
The 990 sits at the bottom of this cabinet with a Carvin Amp sitting directly on top.
It has never ran hot. The fans at each end do a good job of keeping things cool.
Will the 976 generate more heat than the 990 ?

With the smaller box volume I would think that the 976 will feel warmer because the case is so much closer to the electronics. At the same time the electronics may be more or less efficient thus generating more or less heat. Hard to tell but the 990 power draw is 50 watts according to the rear plate whereas the 976 says 40 watts. You have the right setup with fans moving air/heat away from the equipment so I would not expect any issues.

If the numbers on the back panels of the units are correct then the answer is No, the 976 will not generate more heat than the 990. It will generate less.

But, there's always a but, the 976 MAY run warmer than the 990 because it's a smaller package. It's more about heat transfer than heat generation.

I would not expect the differences to be significant in your setup. Overall you are in a slightly better place from a heat generation point. If you have any moving air at all by the 976 then I think the heat transfer will be fine also.

I misspoke , the Carvin amp does have a fan , as it s sitting on top of 990 and a 4' hole cut in shelf above where a Marantz SACD/DVD-A player sits along with the Oppo 980H, the cabinet fan it to the rear of top shelf.
Every now and again I will reach in and press my finger against heat probe to make sure cabinet fan is still working...