If you get an increased noise (esp. grid hum) it may be because you are using a common power supply.
Other than that, and save for extra attention needed about polarity and voltage specs for all devices, it should work.
A 2A adaptor should be capable of driving that - there's probably no more than just over 1A with everything plugged in. There are plenty of adaptors that will supply more than the rated 2A you tried with, Maybe the original is old, cheap, and obviously not up to the job.
A 5A will make you futureproof - always a good thing. The gear will only pull what it ...
Definitely! In fact, Kyuss made their whole style around it.
I don't recommend doing the reverse. You may damage a guitar amplifier with the lower frequencies coming from your bass, especially if it is distorted.
I used 16 ohm horn driver with 8 ohm pc speaker crossover/amp board the Maximum Volume is significantly reduced although it works great see bandmix.com/punkroku for audio sample
MCM Horn driver specs page: https://www.tiny.cc/mcmhorn