Let me know your choice please

MF is great.  I am a dealer of it and they are a strong value.  It is made in Taiwan so you are getting pricing associated with production in China with quality English Design.  They are great sounding and are just a touch forward vs. neutral.  Exactly what you would expect from a class AB Integrated.  DAC is decent.  Phono stage is better than what you would expect.  

The M3, M5 & M6 integrateds are similar in sound profile and ability to replicate detail.  The biggest difference is power output, size of transformers and chassis.  The build quality is impressive.  The remotes are kind of sad and are cheap plastic.  

I have demo'd several Simaudio amps.  They are great.  Sound quality is exceptional.  Can't go wrong with it even driving more forward speakers like B&Ws.  

I am not sure if I would say Bryston's are "too clinical." That usually means so forward it is inching toward bright.  I have listened to them driving a range of speakers and my POV is it is probably the most neutral between the three of these.  Are they significantly better than MF or Simaudio?  Not sure I would say that, they are just a little different.  

Which one is best?  Couldn't answer that.  These are three pretty excellent choices.  I know this is probably not that helpful and you would like to hear these two stink and this one is great.

I have never heard analogue audio and have no POV.  

Bryston cube series amps are really smooth and natural sounding imo. You can also look at used Hegel integrated amps. Simaudio is a good choice, i think they cost more than Bryston for smilarly priced amps. Cannot go wrong with either imo. 

Why?

Lots of information, but what sonic characteristics are most important to you and what exactly are you looking to improve upon over what you have now?

swapping without really having any issues can cause you to make mistakes. what's wrong with the naim? if you like the sound keep it
you are easy pray to salesmen when you don't know what you want
you will have to listen to simaudio or mf for yourself so you can tell if you like it .whether it matches your speakers will be a bit of a gamble unless you audition amps with them
 naim  are great amps if it's just about going to a newer model 
you can try new naim stuff.but since it's without a dac than what's to upgrade ?
sometimes old models sound better. i wouldn't pay 3k for something made in taiwan  .i'm using an arcam amp from the 90's sounds great i see no reason to change an amp that sounds good .once you change the amp to another brand a few things will change at the same time and it will never be the same .