I have just been listening to Beethoven's Eroica while following a miniature score: Eulenburg score at Amazon.
I was paying particular attention to the double basses and I noticed that frequently they went down to the E♭ just below the bass clef (in notation so actually an octave below that). This is not surprising since the piece is in E♭ but it is a semitone below the bottom note of a standard double bass. I was listening the Vienna Philharmonic who use 5 string basses and hence can access this note: Bluray at Amazon. Some other orchestras have basses with a C extension and they could also play this note.
I wondered what would be best if an orchestra (maybe a school or amateur one) with only standard basses wanted to play this piece. The options would seem to be:
Lift each E♭ up an octave. All or most of the containing phrase would probably need to be lifted with it. This would often, but not always, put the basses in unison with the cellos. Where it did, we would just lose some weight. Where the cello part was distinct then the effect may be worse as having the basses close to but not in unison with the cellos could muddy the sound.
Tune the basses down a semitone (either all strings or just the E string). Would this produce a reasonable tone? Would the players cope with it?
For 2, remember that this is likely to be an amateur orchestra so the instruments may be reasonable but not exceptional. Similarly the players' skills would probably not be professional. The mistuned string might be very confusing.
As a variant of 2, we could tune all the strings down a semitone and transpose their part. Provided that they did not have perfect pitch, they may be able to play normally but in E and C♯ minor rather than E♭ and C minor (a retune between movements would not be practical). So, the bass would be a transposing instrument at a minor 9th rather than its usual octave.
Update. I just scanned my score of the 9th (same publisher). I expected to find some low D. I did but I had to scan surprisingly far to find one. I found the even lower C♯ first. In the second movement, I found a low C♮. So, option 2 certainly would not be practical. My scores give no alternative lines even when the double bass has a distinct staff from the cellos.