Soundstage gone

My usual comments: this is too extreme to have anything to do with most audiophile "tweeking around the edges" suggestions.  Either a) something is broken in the cart, b) it is set up (alignment) incorrectly, c) the wires are somehow wrong (this would do it!!!) or similar.
Start with the basics.  The image is very dependent on the alignment since the stero signal must be decoded from two orthogonal squiggly grooves. If your cartridge is not truly perpendicular - poof.  If the VTA is off it is reading/smearing two different "time stamps".  Etc. 

G

Forget about what you heard at the store. That is always a poor listening situation. All you know for sure at the store was both channels were working. The problem was there also, you just did not notice it. 

Iviv,

Have you checked the cartridge connections yet?

If you do not know how to do it, take it back to the dealer and have them check it.

At first read of your original post; a swapped connection, in one channel at the cart, was my thought, which was addressed in the first response.       Crosstalk, caused by incorrect azimuth setting, is another ambiance information (sound-stage & imaging) killer.     That you state, "The sound was amazing.....", would seem to eliminate a lot of other possibilities.     https://www.gcaudio.com/tips-tricks/cartridge-alignment-the-basics-explained/     more suggestions: https://www.sound-smith.com/how-adjust-your-azimuth     Then again; you’ve changed a number of things, at the same time.      In my experience; virtually everything within a system can influence the recovery of recorded sound-stage information, which is some of the most delicate/elusive.

Iviv, Initially I took your post literally, i.e. the only signal you got from your new system was in mono. Upon re-reading your post, and subsequent posts, I am no longer feeling  as sure. In fact I'm starting to think that perhaps you might have a greatly reduce soundstage - the difference is important so perhaps you could better define what you hear. One thing you can do to clarify this is to play a disc and, while doing so, simply press your mono button putting it in mono and see if there is any difference at all. Also switch your speaker connections i.e. left side to right side etc. It would certainly help put your problem in a more defined area of inquiry, i.e. cartridge function issues, cartridge connection issues, set up issues, or just plain old break-in (so much new stuff all at one time). 

Good luck.