Bill_in_pdxThe Blue Point #2 is a high output MC cartridge and should be plugged into the MM phono section of your Cambridge 640P. The MM section has plenty of gain for a 2.5mV output. I used a cart with 1.6mV output into the MM input of a 640P with excellent results.
The Sumiko Blue point #2 Cartridege...moving coil.. output voltage is 2.5mv. From what I gathered that this is too much. That the gain factor would be around 74 with .5 sensitivity. The result would be a high level of hiss.
...Should I even be looking at MC cartridges vs MM's??
I just Googled for your HT-550 and you might find this forum discussion enlightening. Notice that three cartridges mentioned as good matches are the Goldring MM series, a Sumiko Blue Point, and (ta-da) the Grado Gold.
Anyway, it looks like your Sumiko will work fine with it; just plug it into the MM section of your 640P. If the result isn't very compelling, try mounting your Blue Point to a Sumiko headshell. It adds a little weight to the tonearm's effective mass and may be a better match for the BP's compliance.