GarageBand employs loops as one way of creating drums, but you're right, it is very limited especially if your interested in keeping up to date with pop music (trap, EDM, etc). There are a couple of solutions:
This was originally a feature with Logic Pro X and has since migrated to GarageBand (OS X only). As Dave mentioned in the comments ...
Another tip in GarageBand is to set up the Drummer to play something close to what you want. Then you can copy that track that it creates into a new Software Instrument Track. You can then edit that as you would like. This gives you a lot of flexibility, but also gives you a nice full sound to start off with.
Regarding destroying the amplifier, this is not going to be a problem with the Looper. It is designed to be in line with a guitar or a keyboard, and you can set the volume levels of your loops.
You shouldn't worry about or output impedance either. Input is 1MOhm, and output is 100Ohm - which are fine for this usage.
You may not need the mixer - the looper ...
The Harman Canada Engineers who developed the SDRUM also clandestinely included the ability to sync MIDI devices to the SDRUM via the JamSync connector. The SDRUM must be the master since it only sends these MIDI commands. This is not considered an official feature since it was initially included for internal use/testing/future dev only. It requires a custom ...
I suspect you haven't looked very hard yet because there are lots.
There are cheap but not bad ones like these:
and more expensive digital drum kits
I like the ...
I think I am getting a better idea of what you want to do following your comments!
It would be a good idea to make sure that the drum machine that you choose has good playable pads - a lot of people only use the pads on drum machines to sequence things in step time or play simple parts that are then quantised later, so the pads aren't always that good.
In the Sub 37 MIDI menu, try changing the MIDI In Channel setting. If it is set to All, trying changing it to 1. If that still responds to DrumBrute notes try changing it to 2. If it is already set to 1, try changing it to 2.
Clock sync should work even if the channels don't match, because clock messages are not channel specific, but note messages are.