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Not too late at all - lots of useful advice. I had started to suspect the phono stage, I have a Chet Baker record that produces obvious sibilance so I’m gonna A/B test it against a digital copy. Started reading about gain structure but my brain got full and I had to go to bed.
I didn’t know you could just give your room dims to REW - that’s really handy.
dbr-
Room dimensions and speaker positioning including a sub if you're that way inclined. It's pretty accurate but it's only got to calculate the length of a frequency and work out where it will overlap itself and increase the db's.
Gain structure is very difficult to get your head around. It's part of the game that it's so easy to plug components together without understanding their individual specs and they'll work fine, if you want optimal performance you have to understand what they all need to function at their best.
If you are running your phono amp into a pre amp then that could easily be adding some volume which is pushing your power amp input a bit too hard.
I am using digital speakers and I trialled them against an all analog setup, the all analog was nice but I needed to eq for the room modes so it's move or use an eq for me.
Airhead
Kind of late answering but I do use a specialised mic which has a factory calibration file so that a flat response can be calculated. I use REW to make convolution filters that can be used by Roon or HQPlayer in my case. It's all small gains but I've stopped fiddling for the last 6 months or more so something must be working!!
When I had problems with high end harshness it turned out to be the gain staging between the power amps or digital speakers and the sources I was using. Make very sure that you are not sending too loud a signal to your power amps. It's worth knowing rather than guessing what the optimum headroom from your source to power amps is and even measuring the voltage of a reference level sine wave.
If you don't yet have a test disk I would recommend getting one, you can test your phono amp output to make sure you don't have a strong resonant peak in the upper frequencies. Certain cartridge/cable/phono amp combos will do that.
Sounds like you also have standard room mode issues in the low end. They are so predictable you only need to enter the room size into REW and it will accurately predict the frequencies.
I'm guessing the 48khz sampling rate is where the minidsp is economising.