You got your answer for a good starting place, an integrated with home theater bypass. And yes a decent quality integrated will sound much better than the "pure direct" on your AVR.
The amp and preamp both matter, but the preamp has more to do with letting the sound from your source get to the speakers in my opinion.
Some other things to think about... better feet for your SL 1200 if you haven't done that already, upgrading the Puffin, and perhaps your cartridge, which you didn't mention. The Puffin is really nice at it's price, but you will get a fuller, richer, and more dynamic sound with a better phono stage. I use a Pass Labs XP-15 with my SL1200 MKII. The Hana SL has been the best sounding cartridge I've tried on mine.
Soundstage will have more to do with your speakers and your room. I haven't heard the LS50, but they are supposed to image very well. Have you experimented with placement? Have you done any room treatment?
The amp and preamp both matter, but the preamp has more to do with letting the sound from your source get to the speakers in my opinion.
Some other things to think about... better feet for your SL 1200 if you haven't done that already, upgrading the Puffin, and perhaps your cartridge, which you didn't mention. The Puffin is really nice at it's price, but you will get a fuller, richer, and more dynamic sound with a better phono stage. I use a Pass Labs XP-15 with my SL1200 MKII. The Hana SL has been the best sounding cartridge I've tried on mine.
Soundstage will have more to do with your speakers and your room. I haven't heard the LS50, but they are supposed to image very well. Have you experimented with placement? Have you done any room treatment?