Are you sure the problem wasn't due to a defective SpeakOn plug on the speaker cable? There are a lot of Chinese knock-offs, counterfeits, copies (call them what you wish) that are awful. It's one of the biggest problems we see with regard to "defective amp" claims, yet it has nothing to do with the amp!
It's no different than the intermittent 1/4" plug problems due to undersized Chinese sleeves that aren't 1/4" but either a (poor) metric approximation or crappy tooling. Then there's the 1/4" plugs that barely fit the jack and get stuck tight... again a claim of defective amps yet we have to cut the connector off the plug... lo and behold the plug is greater than 1/4" diameter. There are standards for a reason, it's not the amp manufacturers (at least those who use quality parts) who are to blame as we can't control the quality of accessory products that some players are willing to accept.
Save yourselves the grief, use only genuine Neutrik brand SpeakOn plug. The difference is night and day, really. Crappy parts are just that, crappy, and it doesn't matter if it's 1/4" or SpeakOn.