Hi-Fi & stereo tech

Both things are true at once, which is why the argument never ends: the speaker changes a little, and you change more.

You can stress an amplifier with a harder load than it is designed for. Speakers, on the other hand, are almost never damaged by impedance itself; they are damaged by clipping from a small amp driven too hard. Keep those two facts separate and the whole topic gets calmer.

Bi-wiring: skip it. Passive bi-amping from spare AVR channels: try it, it is free. Full active bi-amping: another world, not a weekend job.

A DAC converts digital audio to the analogue signal your amplifier and speakers use, and every digital device you own already has one. So the real question is not whether you need a DAC (you have several) but whether a better one would be audible in your particular chain. Usually the answer is not yet.

You are missing the phono stage. Faint, thin and bass-less is exactly what an unamplified cartridge sounds like through a line input, so nothing is broken.