Best upgrade option for this system?

I can't disagree with upgrading the preamp. Your amp looks to be the strongest part of your electronics. This is not a good thing, because not only is it missing what it can do by not getting better musical information sent to it by the source and preamp, it is also actually revealing the weaknesses of other two in an undesirable way. Always better to have your source and preamp/phono stage outclass your amp than the other way around.

The test will be valid enough for a rough idea. If you like what you hear in the shop, try to arrange to take the Bryston BP6 or CJ PV10 home to find out for sure how you like it in your own room and full system.