Given your requirements, I would go with a Speed Queen set.

I thought I read that speed queen was not good for 2nd floors/upper floors? I am in the same situation as op so will watch this with interest. I don't have budget for miele am considerIng Bosch LG and ???

I might be putting too much stock in Consumer Reports, but their ratings for the Speed Queen models (Speed Queen AFB50R and AFN50R) for noise and vibration were very low. Do people have experiences to the contrary?

Any other models that might fill the bill?

The Speed Queen machines are very nice. But quietness isn't something that concerned Speed Queen when they designed these machines for the commercial market. I have a set and they are about as quiet as the LG's they replaced.

MRB

I have a second floor laundry closet and own Whirlpool Duet w/d. They are extremely quiet and when I close the door you cannot hear them in the next room. As for reliability, I've had them for 5 years without a service call. I would recommend them based on my experience.

I second the whrilpool duet w/d in second floor. we have the same and the noise level is manageable. The only issue i had was carried over from the old owner so may be user related. Other then that whoever see this w/d loves it.

The biggest problem you may or may not have is the construction of your home. There is always going to be some vibration from a stacked set, much less a regular side by side. If everything is very solid, you are probably ok but if the structure is at all bouncy it will translate into the walls and be loud.

There are any number of things you can do, and there are any number of options for washers but the structure will be the limiting factor.

Check out stackable Samsung with VRT (Vibration Reduction Technology).

Here is a link that might be useful: Samsung VRT