Hi guys,
This is my first post as a new member.
An interesting discussion!
Good points have been raised my everyone.
I agree that it's probably not good scientifically to simply replace ICs willy-nilly, hoping for a substantial sound improvement, although I have done this with several channels and the master section of my 488 MkII, changing the original ICs (4560?) for 5532s. It seems to have been successful with no issues a couple of years later, and sounded subjectively "clearer". I did not attempt to measure for an improvement in noise levels. At the same time, I upgraded all the electrolytics associated with the ICs, bypassing them each with a 1uF polypropylene cap (as suggested by a veteran technician on a forum similar to this one). This may have actually delivered some improvement in sound quality over the decades-old original caps. Doing this was a bit of an experiment, and does not mean that I recommend it, especially for those without many hours to spare or an abundance of dexterity!
Now that I have upgraded to a 238 connected to an M-208, the 488 will sit idle most of the time.
I definitely think that a lot of people consider chip-swapping a shortcut to improved noise specs or a better sound, but in most cases it will probably not yield a lot of improvement alone.
As mentioned, matching the impedance of the source to the mixer will have an influence on the SNR, and these are approaches that should probably be considered before "IC-rolling".
My M-208 is pretty quiet to the ear, except when connecting a valve mic preamp and a low output ribbon mic. In this scenario, the trim is set so high that I am hearing a little hum and hiss. I will be re-capping the power supply and then will look at a sensible way to reduce any remaining spurious noise. I am sure the mixer sounded fantastic when new, so getting it to work as new is a better plan than swapping parts for a possibly unrealised improvement, based on some ill-informed amateur comments in forums.