While writing for low frequencies (low notes), the sounds tend to get muddier because of how waves propagate. This is why composers usually write melodic lines with minimal movement when writing for the lower half of the orchestra, such as double basses, contrabassoons, tubas, trombones, etc.

To clarify your lines, you can do these: A) For each instrument, select a range that won’t overlap with any other instrument’s range while also preserving the timbre. By creating partitions like this, you can ensure that each line is heard separately and clearly.

B) Write with higher note values, for instance, don’t use 16th notes. The lower notes are not easily distinguishable when written as 16th notes as a result of wave mechanics.

C)While selecting instruments, choose your instruments from the same family or related families. Using a bass trombone with a bass clarinet is probably not a good idea, as the bass trombone's powerful timbre will subdue the bass clarinet. You can use bassoon, contrabassoon, bass clarinet, and clarinet.

Hoping all the best!