Soundstage gone

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.

You might need to go with an external phono stage that will accommodate MC. Then you can rca into any other input that is open. Vincent makes a nice tube phono pre that would work.

I know this is going to sound crazy but is there a way that you can get another cartridge from the dealer and see if there is any improvement?  That way, at least you have reduced one of the variables in this equation.

OP feared the outboard phono pre would require "putting two phono stages together." This is exactly what it would AVOID. As sgreg1 points out, outboard phono pre should enter AUX or whatever else is available, NOT the amp's own phono stage. If this doesn't clear up the trouble, you've ruled out the diagnosis of defective phono in the amp.