Jrd351 nails it. Here's some anecdotal evidence from my Threshold journey:
In the early 90's I phoned Threshold to ask them which new Forte power amp they'd recommend. To my surprise, Threshold suggested a used Threshold S/300. My contact deadpanned, "you should be able to find one, they were quite popular." Sure enough, shortly thereafter a S/300 Series II appeared in the local paper and I got it. Paired with a Forte F44 preamp, the bass was just a tad slurred. At top volumes, high pitched intruments like soprano sax, flute, upper register piano and violin sometimes sounded screechy. Trying to solve these problems, I replaced the large can caps in the S/300 with equivalent new OEM Mallorys. The sound became a little smoother but it didn't help with the distortion at extreme levels. Then I switched to a Threshold Fet ten/hl preamp and all the problems vanished.
Search everywhere you can, get a handle on the different iterations and you'll figure out which model is right for you.