Stacking gear no more rack space does it affect sound quality

Well at least you know what MTBF is. Does that mean if you operate something below its designed temperature span, it will NOT last longer? I have seen a lot of poor engineering in my day, one common one is REL's R8 failure in several of their subwoofers. Another is using polarized 35 vdc capacitors in a circuit that exposes them to 25vac rms repeatedly. They never made it close to operate to their designed MTBF. Many manufactures including ones that designed equipment for military use, have poor designs, yet somehow they get qualified! I'm pretty sure in todays hi-fi audio world, manufactures use inferior designs and components specifically so they will fail and must be replaced, just to generate more revenue. There are so many examples of this in products from the automotive and appliance industries. Shall I go on? If you operated a piece of electronics in a narrower temperature band than its designed operating temperature specification, it WILL last longer!

Does that mean if you operate something below its designed temperature span, it will NOT last longer?

What component are you referring to? Silicon devices or passive components? As I am sure you know the fabricating processes of silicon devices have improved tremendously and are far more reliable than some years ago. Aging of silicon is so slow that in reality is not an issue anymore, AS LONG AS the parameters (including temperature range) are obeyed. Again passive components should be chosen in a design to operate within their temperature range among other parameters, that is why you have temperature ratings. You wouldn't use a 70 degree C cap in a design where you know that the temperature will rise to say 80-90 degrees.  Same goes for the voltage rating although you indicate that in certain cases that has not been adhered to. I do not know what REL use since I am not interested in their products.

Many manufactures including ones that designed equipment for military use, have poor designs, yet somehow they get qualified!

IMO Standards (ISO, UL and similar) have nothing to do with how good a design is, all they prove is that a consistent process is followed within the cycle of a product. You may have the crappiest of spec and still get markings as long as you produce the same old rubbish.

I'm pretty sure in todays hi-fi audio world, manufactures use inferior designs and components specifically so they will fail and must be replaced, just to generate more revenue. There are so many examples of this in products from the automotive and appliance industries.

You are touching an totally different subject. Really? You do appreciate the complexity of modern electronics, don't you?  And are you telling me that a reputable company will KNOWINGLY design a bad product? I like to see how long the said company will last in the marketplace. There is a difference designing something to a low budget to designing and producing a bad product on purpose.

If you operated a piece of electronics in a narrower temperature band than its designed operating temperature specification, it WILL last longer!

Once again, I am NOT suggesting to operate components at or beyond their limits or their absolute maximum ratings but it is safe to use them under their normal operating conditions whether that is temperature, voltage, current, frequency, load, etc. Of course I am assuming a properly designed component, not something made in a garage...

sometimes you have to do what you have to do.  electronics cases are stronger than you think.  
at the market you wouldn't put canned goods on top of bread so a little common sense stacking components goes a long way.

make sure to never cover any ventilation slots.   
make sure the feet of the upper components are fully resting on the surface of the supporting component.  
stack with heaviest / largest footprint on the bottom.  
with solid state gear you should be fine.  i do not believe you will notice any audible issues.  

Components vibrate internally and that transfers outside the boxes - stacking them adds to each other's vibrations synergistically. Even phono stages vibrate, and because they deal with tiny signals, even small vibrations impacting them are detrimental.

Plus you have the ventilation issues mentioned above. Avoid stacking to whatever degree possible. If you can't, remember what the great CE says...."A man's got to know his limitations".