why do expensive amplifiers produce a better soundstage

Soundstage is an acoustic/psycho-acoustics concept...An amp can contribute but cannot create it without working with the room acoustic parameters and without working with the psycho-acoustics parameters..

No more than a room and your brain/ears can increase the details of a digital format but only contribute to his resolution  and perception...

The really expensive ones” because they address both electrical ground and mechanical ground issues. . 
Goldmund, Gryphon, CH Precision, dartZeel off the top of my head. 

@yamaho 

I don't believe this is true anymore, I've been playing with the budget equipment I mentioned up-thread and recently added and inexpensive Teac turntable that uses a very nicely designed SAEC knife edge baring tonearm and an Audio Technica VM95e cartridge with a bonded elliptical stylus through the phono stage of the Rotel a8. I'm playing these with a pair of Dali Oberon 5s that I have mentioned elsewhere in the forum and are listed in my profile. The results are quite remarkable with my digital frontend and Magnum Dynalab tuner and equally so with the turntable of which I'm planning a stylus upgrade to the AT-VMN95ML in the near future. I think my positivity will be verified!

One video on the internet suggests that any amp costing over ten thousand, your money is then going towards cosmetics. On the other side, the McCormacks aren't expensive although they could get there depending on upgrades are known to be soundstaging champions, even in their stock form.