Point taken. My initial bass correction was indeed based on the RoomFit analysis done by the Amp Ultra.
But that only assisted me in locating the critical frequency spot. The extent of the correction was then dictated by me.
I’ll run another RoomFit analysis with the new setup tomorrow. My guess is that the result WILL be different. We‘ll see.
		
		
	 
I'm pretty sure that the PET1200 DM is on another level and that most certainly there can be dynamic differences, in particular when some current is really needed. Just don't over interpret initial expressions when you first plug in a new (and expensive) piece of kit. It will always sound "totally different" and "better beyond compare" ... when simply "better" is actually quite an achievement.
Once again, I'm sure that this amp is as close to the state of the art as you can get.
	
		
	
	
		
		
			The sound below 47Hz is there, but it's a bit muffled. Should I add an external microphone to correct it?
		
		
	 
By "muffled" do you mean it's still lacking level or power? Or ist it rather like it appears somehow slow and not as punchy as you would like it?
An external microphone (line e.g. the Dayton Audio iMM-6C or the miniDSP UMIK-1) will definitely yield more precise results. First because of the better quality of the microphone capsules, but probably even more so because they come with a calibration file you can upload into the WiiM Home App and because they are tailored to be used for measurements, not for recording spoken words.
The point 
@Wiimer made is that your iPhone mic could pick up less bass below ~47 Hz than is actually present. in this case RoomFit would attempt to further boost the bass until the measurement looks flat. A true microphone would show the real amount of low bass, which could be higher than what was measured with the iPhone mic.
To further complicate things, WiiM do attempt to compensate for known characteristics of iPhone microphones. This is in fact a microphone calibration, too, but it's not individual. It attempts to correct for a class of devices. Ironically, it is also possible that WiiM's generic iPhone calibration overestimates the loss below ~47 Hz for your individual phone, so RoomFit ends up adding too little.
The best way to get out of this dilemma is to get an external mic with individual calibration file, indeed. The iMM-6C is probably the least expensive serious contender. Get one, if you can.
Back to my initial questions:
If the bass doesn't feel lively and punchy, you might actually miss more bass in the area around 100-120 Hz (which could be muddied by "too much" bass below 60 Hz).
If you really miss the authority in extremely low bass notes (not even present in most pieces of pop music) there could be too little low bass. I think that this is less likely the case, but better measurements can reveal the truth.
PS:
HouseCurve uses a generic calibration for uPhone mics, too, but it's known to be quite accurate.
Another PS:
Since the iPhone 12 is lacking a USB-C port, you will also need the Apple camera adapter to use any USB mic. 
