Like Nick, I compared stock with SL and then Sarum Supers and although the jump in performance from stock to SL was significant (and if they were the only option, you’d definitely buy them), the move to Sarum Super was at least as big an improvement again. The fact that they were much cheaper was a welcome bonus. In fact, for similar money to SL you can get ChordMusic and that’s a whole different ballpark...
FWIW, I visited my dealer with the expectation of replacing my 300 with a 500, just at the time when the 300DR was out and about but the 500DR wasn’t yet in stock. I compared my 300 with 300DR and it was a no brainer, better in every way, but then comparing 300DR to 500 was less clear cut. The addition of ChordMusic Din/XLRs to the 300DR made it a clear win and I ended up going that route.
I lived happily with that combo until a dealer and then home demo of 500DR, the latter with Music in place, demonstrated the clear and substantial improvements offered by the 500DR, but that’s another thread. Given the choice again, I’d still do Music first as what it does is so profound.