Just starting to learn

Which is the better preamp? Well, you could judge by reputation, build quality, sound or any combination of the three.

If sound is what you care about (and I think that's the case for most folks here), then do an A-B test. Set up the system with one preamp, listen for a while, then swap in the other preamp and listen to the same selections one more time.

One of the preamps will sound A) about the same as the other, B) different from the other or C) better than the other.

Well, I've heard that the lower-power Adcoms in that series (555-545-535) sound better, so I'd try the 545. I'd then sell whichever amp I didn't want, unless I wanted to try biamping. Biamping with those two amps should be easy, since their gain is probably identical. I don't know if your speakers have double (biwire) input connectors, though.

After that, biamped or not, I'd start upgrading with the source. I'd get the best one I could afford, then the best preamp, and so on downstream. Once I had the components I wanted I'd start tweaking with cables.

For specific recommendations, you should post what it is about your system's sound that you'd like to improve. A lot of people would be happy with the gear you have and wouldn't see the need for any upgrades. Maybe that includes you.