Tubescreamers seem to be a love/hate thing for bass. Some say they work well and others disagree. With a blend they can work very well. On BYOC's site is does say that the 855 has more low end than a typical TS pedal, so you may just want to build it as is and see how you like it.
As for bass mods, check the BYOC forum. They are pretty good about having a lot of well documented info with pictures.
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