Amplifier distortion help please

Hope someone can help. I seem to be having a weird problem, in that I keep getting distortion on the left channel of my system. This does not happen all the time, but usually, once it starts, the amp is damaged. It occurs from either of my two turntables or my Cd player.

So far I have had my Musical Fidelity 'The Preamp' in for repair, one of my Sugden P128 monobloc's, and now the amp I bought for a spare whilst the Sugden is in for repair, a Marantz PM-54 is distorting on the left channel. I also tried a friends DPA power amp and this distorts too!

Most of the time I have been using a pair of aged Mission770's, but changed to my spare pair of Celestion DL12's and left channel is distorting, although this was after it started distorting with the Missions.

Talking of the missions, the smaller coil on the crossover board looks like it used to be connected to the negative speaker wire post, but the wires stop just short of this on both crossovers.

Added later: Yesterday I turned on my system and left channel was distorting badly with very low volume being produced. I changed the left speaker and it then sounded fine. Replaced with original speaker and still sounded fine - ie no distortion ?????????

Here's hoping,

GAry