Disklavier

I have a brand new C3 with a DKV Mark IV.  It is 2 months old.  
There seems to be 2 faults with this DKV, Yamaha says not so.

My question: Can anyone out there with a Mark IV please tell me whether the digital piano in quiet mode is a close tonal approximation of the acoustic piano - or is there a big difference - like chalk and cheese?  None of the dealers where I live have a Mark IV on display.

Background:
The first fault appeared whilst playing in Quiet Mode with headphones.  The digital piano suddenly stopped playing.  The only way to get it working again was to power off and on again.  A string of I/O errors were reported on the screen of the PRC100.
This has now happened a total of 7 times and Yamaha denies there is a fault, claiming that because I have connected a laptop to the USB port and played some MIDI files on several occasions, that this has somehow upset the settings of the DKV.  In none of the instances when the keyboard drop-out fault has occurred, has the laptop been connected to the DKV.  I have agreed with Yamaha that we will wait and see if it happens again.

2nd fault: I continued to play in quiet mode without headphones and immediately noticed that the digital piano had lost its "guts" -playing loudly does not produce near the volume that it used to.  The tonal quality (in my opinion) has also suffered.  Before this occurrence, the digital piano in quiet mode was a very good approximation of the acoustic piano.  Now it sounds like a $1,000 cheapie. The tone is a flat tone with no harmonics or overtones that you can hear from the acoustic piano.  This is somewhat subjective - when I first got the DKV I was amazed at how good the digital piano sounded.  Now I am embarrassed to show it to anyone.
A Yamaha rep has called but can't get it back to what it was (in my opinion) before the fault occurred.  He says it sounds ok.  Hence, my question above.

Kind regards