One sound cancels another sound out.

Most of the bluegrass is in your thumb, you play the bass strings with your thumb, and leave the rest of the job for the rest of your finger. Johnny Cash and Mark Knopfler are the two dudes who opened my eyes to bluegrass rythms. Also Johnny Can help you a lot on those bass tones try singing daily as deep as you can but don't stretch it out, just as much as you feel comfortable everyday and after a while you'll get the Barry White kinda bass sounds. And for the bluegrass thing just check the internet out there are tons of high quality in depth reviews of the thumb picking style, and try playing to a four bar backing drum track and follow it up with your thumb. And that's the rule of the thumb.
Common Bluegrass Flatpicking Licks Guitar Lesson Tutorial

You can use it, just keep the volume low. The amp doesn't care what yu play through it, but the speaker isn't designed for loud bottom end.

If you have already tried a different cable and it didn't work, then the issue might be with the bass drum pad itself. You can try the following steps to troubleshoot:

1. Check the connection: Make sure that the cable is firmly plugged into both the bass drum pad and the module. If the connection is loose, the signal may not be transmitted properly.
2. Check the sensitivity setting: Check the sensitivity settings on the module for the bass drum pad. It is possible that the sensitivity is set too low, and the pad is not registering hits.
3. Check the pad: Check the bass drum pad for any visible damage or wear. It is possible that the trigger inside the pad is damaged or malfunctioning.
4. Reset the module: Try resetting the module to its default settings. Sometimes, this can fix issues with individual pads.

If none of these steps work, it may be necessary to open up the pad and check the trigger mechanism inside. If you are not comfortable doing this yourself, you can take it to a repair technician.

Sometimes there is a resonance with materials that are a part of the case or inside the cabinet that cause this. If the level is very low, don''t worry about it. I have even seen the logo on a speaker grill causing the noise... also debris inside or behind the grille...

His amp probably has lower gain. They vary quite a bit. Also the size of the speaker will affect it a bunch. If his is a tiny practice amp your bass will not generate much noise.

My guess would be something faulty with the cable connector to the kick pad.
Source the faulty link by reconnecting the pad so that:
1. A tom pad triggers the bass drums sound 2. The kick pad triggers a tom sound 3. Don't use cable you use with the kick when searching for the fault.
This should tell you whether it's the pad, the bass drum channel or the cable that is at fault.
Hope that helps!

Turn down the treble gain and mid gain... you don't need it for a bass. Prevent the guitar from "hearing" the output of the amp directly. The guitar strings make a good microphone.

Our human perception of loudness can be way off. You should get a soundmeter and compare. There are some pretty good ones on eBay new for about $140.

Adding speakers may NOT increase the sound much as the power will then be split between the speakers.

If you are sequencing midi tracks you can imbed the patch change in the track and the midi channel assignment and when the sequence plays the info will be sent to the keyboard. Do you have it set to receive Omni?