How to attach speaker in soffit without drilling it?

Ahh, but that's the thing: It's not the subjective experience that matters; it's the objective experience. If you want better accuracy in your mixes, you need better accuracy in your room! The thing is, very very often, we can hear that things aren't 100% right, but we can't put our finger on exactly what it is. You might notice that your mixes just don't sound the same when you play them back in your car, house, church, office or wherever, but you can't actually figure what it is... even if someone put a gun to your head, you would still not be able to say what exactly it was that didn't sit right... and that's because the human brain just isn't capable of discerning the exact problem: even though it is aware of it, it just doesn't have the ability to tell you that the problem is that the left speaker is 2.7 dB louder between 590 Hz and 870 Hz, and there's a difference in phase above 2 kHz, plus there's a reflection that arrives at 7.3ms, and an SBIR dip at 73.2 Hz.... All of that can be clearly seen in the acoustic test data, and some of it can be fixed easily... but your brain is just not capable of hearing what the problem is, other than to say that it "doesn't sound right"...