A good original Selmer 'Soloist' (or 'scroll-shank' soloist type) in the right tip size can be a beautiful thing. I've had a couple of original facing soloists/ scroll shanks (C**, and E), and one re-faced one, and currently am using the orig.'scroll-shank' E facing (.055) which works well for me in giving a warm, rounded non-edgy tone without nasal quality: even tone and in tune, nice resistance, and easy to play.....qualities I most was looking for in a soprano piece......this on a modern Yanagisawa sop sax. (I use a vint. Buescher large chamber mpc, refaced (.057) by Joe G on my vintage '20's curvy Buescher, and King Saxello horns.) Added benefit for me of the Selmer 'scroll shank' mpc. is that it works well with Legere Signature synth reeds. I've other excellent choices in sop mouthpieces I could be using: a vintage Slant Link, a NY Meyer, Drake 'Son of Slant', Morgans, etc.
For sure, one major difference between the 'Son of Slant', or Morgan (assuming the 'Vintage') you currently are playing and liking, is that the chamber size on your mpcs. (son of slant, morgan) are larger.....the Soloist has a small chamber with a squeeze throat.....so these play very differently. A question would be.......what are you looking for different than what you are already getting with the current mouthpieces you're using? You will really need to try a smaller, squeeze throat design before you will know if this will work for you. And the point was already made that a good scroll-shank or Soloist will cost you $$$ (more open tip pieces can be pricey indeed). There are perhaps a number of similar designed modern pieces substantially less expensive. Actually, it is perhaps a good suggestion contacting Joe G. of 'Soprano Planet' to discuss what you are looking for and what may best work to meet your needs. Good luck.