If the change happened suddenly, check the battery, EQ, pickup settings and amp before blaming the strings. If none of those changed, the strings may simply be at the end of their life.
The low B is often the first string to lose definition as a set ages. If it's gotten muddy while the rest of your setup is unchanged, it may be time for a fresh set.
A new low E can sound different from the other strings at first. The lowest string produces different harmonics and takes a little longer to settle. If the difference lasts after several playing sessions, check pickup height, string installation and setup before assuming the string is defective.
Older strings aren't always bad. Many bassists prefer their warmer sound for jazz, blues, soul, reggae and vintage styles.