Wow, that is a hard one. You have two pieces of high end audio gear and a couple of budget components... and looking at another budget component. I guess, I would save up and buy a used DAC... something that would have originally cost well in excess of $5K. Or maybe as a minimum a Schiit Yggdrasil. Then upgrade your preamp to a much higher level. You have really potential here... but I wouldn't recommend tiny upgrades.
Basically your speakers and amp have much greater capability than your other equipment. The real key in getting the most out of your equipment is matching capability of all the pieces. So, you have tremendous opportunity to get better sound quality by upgrading your other pieces.