As the OP, I’m interested in your case. The impedance of your speakers is not unusual.So two requests, please: 1. Do you have specs for the amp? 2. Measure (using REW) the in room FR and leave both measurements on the same graph. It doesn’t matter if you use your laptop microphone because we’re only looking for differences.
I can't find more A21 measurements, other than official statement of +1000 at 20 Hz.
In soundstage, they do test other Parasound amps, including A23 and JC5. I will assume A21 is something in between A23 and JC5.
You can get measurements at
https://www.soundstagenetwork.com/index.php?option=com_content&view=article&id=17&Itemid=141
I don't think this is something that could be measured with REW. I have another thread where my DAC got channel imbalance of ~0.2db and offered to measured with REW and was told REW is not accurate enough, period! So, not going to spend time doing REW measurements for overall balance changes that are most likely also in that range. Just going to give my observations and leave it at that.
Another issue is personal sensitivities to minor sounds changes. Not everyone is as sensitive. My sensitivity is pretty high. See my screen shots of my online blind tests here:
https://www.audiosciencereview.com/...ds/how-good-is-your-hearing.20417/post-692530
Edit: another data point. My 10 ft 10awg cables are connected via bare wire method. My previous generic 12 awg cable are terminated with banana plugs (cheap ones from Amazon).