everything sounded great until the upgrade

wine improves all systems.

With the empty bottle you can use a straw for the neck and experimenting with a primitive Helmholtz resonators...

I begin my first one like this...  😁😊

If you liked the sound of your modest system.....that bis all that counts.

I had MF M6i (before built-in phono stage si model) and Revel F32 speakers sounded great with it. Also, if using the built-in DAC, I suggest buying a stand alone DAC. Built-in DAC is weak link and Ayre QB-9 sounded amazing with my MF M6i. IMO, Musical Fidelity is very good. M series much better than A series (M series replaced) and more w/ch achieve better sound with MF gear. If I did not swap SS for tubes, I probably would still have the MF M6i.

I would suggest seeking new cables with larger diameter conductors. Synergy between the components in the modest system was probably achieved. Changing the component requires adjustments.

With Interconnect and speaker cables and any conductors in the components themselves (traces on the PCB, internal wiring), at least in the audio spectrum,  a larger conductor will yield more emphasis on the low end of the spectrum and a smaller conductor will yield more emphasis on the  higher end of the spectrum. The sum of the signal path, the matching of all the component should amount to an audio balance that fits your taste.

Maybe the internal wiring of your two amps are of different size and yield different tonal response (high end or low end emphasis). If you can manage to identify the diameter of the isolated conductor in your IC and /or speaker cables, You could try to other IC and/or speaker cables with larger conductors to get back to the tonal response you had with your previous amp.

For instance Duelund offers relatively inexpensive hock up wires of different gauges and compositions, It would be interesting to try them if you can put together the IC or have some one do it for you.   From there you may find the gauge or wire that will make your system synergistic again.

@itsjustme 

Why did you buy something without hearing it first? 

Because it's used, can't hear it anywhere

@mahgister 

design change is less impactful on listening test than acoustic and psycho-acoustic change in the speakers/room relation

Right. But when my room is a given, not many options in placement, my main variables are components and cables - as I stick to the speakers