Well, having owned a 2006 R171 SLK350 for 12 years with the standard audio 20 and standard speakers, I can give a comparative.
Sadly, I am disappointed with the standard Audio 20 with standard speakers in my SLC43
I wish I had paid the extra £645 for Harman Kardon - just to get the 11 speakers / amplifier channels instead of the four(?) that I have. I'm not so interested in the fake surround sound, although I haven't heard it.
There is just not enough bass "oomph" that the reversed 8" door woofers had in the SLK - in the R171 SLK the volume and audio punch was waaaay good enough for fast top down cruising and reasonable audio balance and power.
The SLC just lacks enough low frequency power. Turn the volume up and it is all mid range and treble and not at all satisfying.
It's just about "OK" with the roof up but not good with the roof down.
I have even adjusted the tone controls (Which I don't normally do due to it crudely unbalancing the sound) to increase the bass by +5/10 and decrease the mid by 1/10 and treble by 2/10)
My opinion: pay extra for the amplifiers and speakers if you order a new one and you are in a country like the UK (where you don't have the Premium 1 Package that the US has including the HK Audio and included as standard with the AMG model)
I guess I just have to live with(out) the sound now, unless I go down aftermarket options?