The reason I ask is I haven't played a block letter in years, and the deal breaker at the moment:
Found a block letter for $550, so...
Wirelessly posted (Wondernuts)
5150/6505 have no differences other than the tubes they shipped with and the 5150II/6505+ are also identical.
I have a 5150II and it works a treat. The 5150/6505 is a little more gained and a little rounder in the low end. I tend to leave the resonance knob at 12 I clock myself.
So for $550 you should get it because you can always resell it for what you bought it for.