itunes match quality?

Recently imported my entire music library into itunes, and I've got some really old files from 20 years ago I originally ripped from my CDs that are 128Kbps. Back then that was thought to be "cd-quality," but we obviously know better now. Long story short, itunes match "upgraded" all of these old low bit-rate files to much higher quality AAC files, but the many of the new files sound terrible. I mean, like playing through a wet blanket with cotton in your ears type bad. No dynamic range, no high end, no low end - almost like the low quality file was reprocessed from the low-bitrate mp3 into the aac. I am playing through a high quality receiver and speakers, where it is most evident. Not so much on my computer or bluetooth speaker, so the question is: WTF is going on?

It is also hit and miss, as some files sound great and better than before. But some are obviously much worse. I still have the CDs in question but was hoping I wouldn't have to re-rip them into higher bitrate since itunes match was part of what I paid for on itunes (talking maybe a few hundred titles here). I found another thread about this but was unable to respond there due to the age of the discussion. Sound quality using iTunes Match is terri… - Apple Community