I also had problems with Tascam decks. Those exact ones. Most of all that sound was often in total imbalance. Left & right channel volumes. Sound would be very quiet or blurry.
Instead, Technics RS-TR series decks, I have used for many many years. I have ended up discarding few of those too, but of course none of the consumer level decks are meant to non-stop dubbing for years. However, these have proven to keep sound crispy, clean, strong and loud.
I nowadays (for some years) check EVERY tape on deck before it goes for sale. Just to make sure that sound is clean and loud. If defects are found, I'll check was that one-off problem caused by tape itself or possibly something to do with deck. It happens rarely, but does happen sometimes.