Well, I auditioned an ND555 when looking for a streamer to replace the CDS3/555PSDR.
It’s a super source.
If you are happy to stick to streaming, then swap power amps or possibly better still, consider room treatment.
If you have or would like a record collection, then buy a turntable. There is endless debate about the relative merits of streaming vs. Vinyl as a source. As an owner of a Vertere MG-1, (and former LP12 owner) I would very much doubt that you would find it to be disappointing, so long as it is well set up.
Your question is a bit like asking, “do you prefer chocolate mousse or Picholene olives?”
I think it depends on whether you want to play records. If you do, then get a record player. If you don’t, then you could improve the amplifier. But do you really need to? The 552/300 is a very fine combination.
Are you starting over with vinyl or you do have a collection and like the revival…?
Vinyl can be a rabbit hole especially these days of madness and speculation.
This could be a safe reasoning. Starting now with vinyl could really be expensive and not for the HiFi gear…
And as already suggested, a less than top notch vinyl playback system would not bring you great results. My humble opinion, of course.
By the way, great System