Don’t use kimber cables with Naim amps, this should be avoided due to capacitance / inductance values being the opposite to naca5.
Your 5si is going to get hot.
Naim power amps work best with naim preamps and vice versa.
So ideally with a nap300 this will be nac252/supercap.
This is an amp with an rrp of £8600, plus 252 £7700 plus supercap £5300. £20k is serious money for a seriously good system. Please try to partner it correctly or you’ll never fully realise its potential. It will also sound nothing like the 5si.