System advice

I am sorry to hear that your upgrade did not go as planned. I am not familiar with the sound of Naim equipment, but regardless it is on the low end power-wise for the speakers. I could not find exact specs on the speakers as there are a lot of Monitor Platinum models, but I am guessing that the impedance curve is probably not favorable for the Naim amp. Based on what you feel is missing from the sound, I suggest starting with a new power amplifier that has more wattage, and performs better at lower impedances and is known to have a touch of warmth. Budget will drive your choices of course, I highly recommend looking at the used market here and on USAM.  

I am not the only person who likes Naim digital gear but not their amplifiers.  I would focus on finding a substitute there.  If you can get your hands on other interconnects and speaker cables you can try something different.  Naim amps are somewhat sensitive to choice of cabling, particular the speaker wires.  If that doesn't do it, look to what you changed which you did not like which is the amp and linestage.  The components that affect the sound the most are the speakers, followed by the amplifier and then the linestage.

The only way you can determine what amps will do it for you is to audition some candidates, preferably by borrowing them and using them in your system.  For solid state, I suggest looking at brands like Ayre and Lavardin.  These deliver a sweet sound that is quite different from that of the Naim amps. 

But, my own favorite amplification is tube amps, particularly lower powered tube amps.  I don't know the specific requirements of your Monitor speaker, but, it is often the case that people are satisfied with amps of much lower output than they would normally consider when it comes to tube gear.  I suggest that you look at an integrated amp from Synthesis Audio (Italian).

+1 @akg_ca

Have the Naim pieces had time enough to fully break in? If not,that may be the problem. If it has, then you have a synergy problem. It’s not a lack of wattage. 60 wpc is enough to power the speaker which are 90db efficient. The amp and speakers don’t play well together. Change the amp/pre or the speakers

Personally a fan of Arcam, I feel it's highly underrated so going off that was a mistake IMHO. I’d avoid getting onto the cable merry go round personally. EIther go back to Arcam or consider Parasound or Luxman as good directions.

Go back to the Arcam if you liked it before.