I need some advice on recording

I have two problems
- On the bass cab, the lowest notes sound really really flabby, especially the open E (tuned in D)

I used an AKG D112 and a condenser mic as an ambient mic (AKG perception 170).
Do I necessarily need to couple the cab mic with a DI signal? Or use a compressor? It really sucks

- On the Guitar cab, I used two paired symmetric SM57, one for each cone, sounded fine but I had a weird problem...
At some point in the song, I can clearly hear a phase inversion. I checked if it's a phase problem and it is... I think
I am sure I didn't touch the mic while I played. What the f. happened? I really don't want to split the tracks on Reaper and fix this crap manually

Anyone has an idea of what could be the problem?