Stereophile's budget cartridge survey (a variety of cartridges now in the $50-200 range) of a few years ago (available online as Gramophone Dreams #44) was interesting in that a $9000 turntable/tonearm combination was used for much of the testing. Also on hand for comparison were a 1980's era LP12 with Basik LVX arm, a Technics SL-1200MK2, and a Pioneer PLX-1000. One comment made was, "On the Blackbird-Schick deck, these so-called entry-level cartridges played recordings with a layered, three-dimensional complexity that I thought was impossible at these price points. Quiet musical passages were not only quieter; they were filled with magic little details that got noised over, dulled over, or disappeared entirely on the Pioneer and Technics 'tables." I bought an LP12 with Basik arm in 1985 because with its cheap AT-derived cartridge (looked exactly like an AT95E but with a conical stylus), the Linn sounded better than my budget Thorens TD146 no matter how expensive a cartridge I asked the salesman to mount up on it. So my experience is no, all the sound quality is not coming from just the cartridge and phono pre.
I have no familiarity with your PL550. I don't know if it's possible where you live, but the best way to answer the question would be to do a comparative audition at an accommodating VPI, Technics, or Linn dealer. Your PL550 sounds great to you, and it will perform just the same even if your ears hear improvement from more expensive turntables. One of my favorite observations from a review article was the following in Jason Kennedy's review of the Rega Planar 2:
"Going from the Planar 2 [fitted with the budget Rega Carbon cartridge] to the new Planar 3 with its Elys2 cartridge and multiple other differences does make a good case for the more expensive turntable. You get more power from drums, a lot more fine detail, tighter timing and more precise focus on the various instruments; more is more. But going back to the P2 is not a painful experience because the fundamentals are still the same, the music is no less enjoyable but the resolution is lower. As soon as you stop comparing the two the more affordable turntable delivers rhythms and melodies that are more than engaging."
Spending more can always get you more, but I love Kennedy's statement that "the music is no less enjoyable" with the cheaper package, at least once you "stop comparing the two." I heard a difference between my Thorens and the Linn that I decided I couldn't live without, but that might not be the case for you and your PL550.