Two possibilities come to mind:
1 - The Speedball reduces distortion, but it also changes the harmonic structure of the remaining distortion. You may just prefer the original for this reason - people and their ears are not all the same, and our differences are all equally valid.
2 - We do get used to things we are familiar with. I suggest you listen to it for an extended period, maybe a couple weeks, and then restore the amp to the original and see whether you still prefer that presentation. If you then keep the parts in a box, you can swap in the Speedball again in a year or two to see if your tastes have changed. You will certainly learn something with any of these experiments.