I spent a lot of time with both the 501 and briefly auditioned the 501SP.
Long story short; the 501 sounded terrible. Put over 1000 hours on them with no improvement. I had several friends with critical listening skills over and that session lasted about 15 seconds. Seriously.
I had a shorter audition with the 501SP. Not much better.
It's interesting that the previous US importer eventually dropped the 300 and 500 line and only sold the 700 and F1 series.
It cracks me up how Upscale Audio, the Tannoy importer at the time, trashed the Fyne line every chance they could. They then dropped Tannoy, picked up Fyne and now the opposite; greatest speakers ever. Sheesh.