What is it that you find lacking and need to be improved?
Your current Gold Note phono is an excellent unit in its own right and I don't think you have yet to approach its limits. I would explore upgrading the cartridge first if I were in this situation. From my experience (having gone through this process), I can say that the cartridges make a big difference. I currently have: SoundSmith Zephyr MIMC, SoundSmith "The Voice", Kiseki Purpleheart, and Aidas Blackheart cartridges. I noticed a big difference having moved from the Sumiko Songbird cartridge to the Zephyr MIMC. From the MIMC to the other (more expensice) cartridges were incremental differences. Between "The Voice" and Kiseki "Purpleheart", I'd say there are differences moreso than improvement.
I am sure the Boulder 508 is an excellent piece and should pose an improvement over the Gold Note, given the $3k price difference. Now, the question is whether the improvement is worth $3k, or is the $3k better-spent on a (better) cartridge in the $4k-$5k price point? In this price point, I'd suggest the SoundSmith Sussurro, or the Dynavector XV1s, just to name a few. Personally, I've been eyeing the Sussurro or the Dynavector Te Kaitora Rua, but I am trying very hard to resist pulling the trigger. I just cannot justify for another cartridge given that I already have 4 in rotations.