what cable should I buy for my system?

Back to your original question, I have had good luck with both WireWorld and Acoustic Zen cables. They both use OCC copper and in every case, excepting power cables, they have made a positive difference when introduced to my system. I have yet to delve seriously into power cables, hence the comment.

And yes, you should consider a separate preamp now, and some digital separates down the road. 

I forgot to add, there is a strong used market for both of these brands, you can save a lot going used. 

use anything that is OCC single crystal wire it is far superior to anything OFC at any price, harmonic technology acoustic Zen are quite reasonable.

I have Bowers speakers and they work really well with Transparent co cables.  Similar to MIT cables.  Located in Maine, Great to talk to on the phone to help.

I'm sure you're familiar with the Hippocratic Oath taken by physicians, which simply states: "First, Do No Harm".

Audio cables are passive devices, and at their best, pass signal from point A to point B with an absolute minimum of artifacts.

That oath can also be applied to cables. Because ANYTHING added in the path of the audio signal WILL do harm.

The finest cable is the one which achieves this goal to the extent the state of the art permits.
And, by keeping thigs simple, the cable does not have to be ridiculously expensive.

No hype, no games, no glitz.
No pseudo-scientific jargon to separate you from you money.

As stated in the Stager Silver Solids website: "it is not how a cable makes music sound great. It's about how it allows music to sound great by not hindering its transmission in any way."

Just pure signal. - Like these:
https://silversolids.com/