just because the A300 has a 5.25 inch woofer compared to the SBS 380 (4 inch woofer)
that doesnt mean the 5.25 inch woofer plays better details.
the 5.25 inch woofer has the potential to move more air.. but if the 4 inch woofer has clarity and detail, it would be the winner.
have you tried searching the internet for videos of each subwoofer?
there are usually videos for every subwoofer being shown off.
i dont know which one is better.. but an internet video with audio might help you learn.
maybe you can ask the seller to record some audio and send it to you on the computer?
i'd say pick a song that can prove if the subwoofer has any details.
its best to play the same song on both subs.