Hello,
Whatever you can get a better deal on used out of a DAC or an integrated. I would replace the DAC first especially since you are using a transport. You have really great speakers. When you replace the DAC with something of that caliber and price you will be amazed. The biggest reason people hear such a big sonic change when they upgrade the integrated is because of the preamp section in the integrated. This is why I prefer separates. You buy an amp that is good enough to push the speakers well. As you upgrade the preamp you get a really jump in sound. Plus, if you are running all digital you can run a killer DAC on a good amp especially because these DAC/streaming companies have figured out to adjust the volume from the chip and get great results. I still prefer a preamp with analog inputs even if I am running all digital setup. I run a nice vinyl system so I need analog system. That’s why I like the Lumin P1. It’s a preamp/ DAC Streamer. I just listened to this awesome piece of gear on a 20 year old amp pushing KEF blades. Pure harmony.
I also love the sound of the Ayre QX-5 DAC with the Network/ streamer option.
Another reason I prefer separates is you can upgrade individual pieces of gear and if one thing has become obsolete like a DAC you can replace it. I do know that some companies like MAC and NAD have cards to make them semi future proof.