I am unable to recommend specific desktop speakers for you,
@AfroSnowman, but I do have a few suggestions to consider. I think it would be better to purchase a pair of speakers from a store for a home trial rather than buying them online. When you visit the store, explain to the manager the difficulties you're experiencing with tinnitus and hyperacusis, which were triggered by the Amazon Basics speakers.
High-quality speakers should improve the sound and, hopefully, reduce or eliminate any distortion and sibilance you're currently hearing. You haven't mentioned whether you're using a laptop or a desktop computer. The computer's internal sound card is also important. I mean no disrespect, but connecting high-quality speakers to a lower-quality sound card usually results in subpar sound. That's why I suggest testing the speakers at home first to avoid disappointment.
How long do your conference calls typically last? Do you make multiple calls throughout the day? I ask because prolonged exposure to sound could be affecting your ears. Another factor to consider is the size of the room or office where you take these calls. If the space is small, sound may reflect off the walls and ceiling, which could worsen your tinnitus. Acoustic sound-absorbing panels can help if reflected sound is causing a problem.
Do you watch TV or movies at home using a surround sound system with a subwoofer? If so, this could be making your ears more sensitive to sound without you realizing it.
All the best,
Michael