I have installed a MM OT on my DSL50 for some years now and it makes a audible difference.
It makes the amp more tight and 3d and not as harsh, like it sweetens the highs a bit. The effect is noticeble immediately.
I have lend my amp to a Paul Gilbert clinic here in greece and there was also one other DSL50, which was totally stock/brand new, both set by his engineer to be identical (minus the MM) and its was apparent to Paul from the beginning that there was a difference in sound.
Hope this helps.
Stelios