I have done lots of research, i started and still have with a submersible pump.. Beckett brand pumps. I have had them 2 yrs, and this past yr I have found the pump to be unreliable. I am going to stay with the submersible until it dies (and until after the wedding im on lock down with spending)
A well done external setup is far superior that any setup you can have with a submersible. As for bottom drains, i think if designed well, they are great. I myself will be building a retro bottom drain as soon as I get my external.
As for noise, seriously? these arent pool pumps that you can hear from 3 houses away. Do your homework, more often times than not you will see successful koi hobbiest have BD's and external setups. I'm speaking from experience of having a submersible pump setup. While its very adequate, it could be much better using an external pump setup
people here are quick to jump on people here when they speak of something they are not familiar with...