[R-390] Spectrum Analyzer

[wa4aos at aol.com]

 Sorry for the OT question but I am trying to make a decision regarding another Spectrum Analyzer for my shop.
I presently have a very nice 8566B SA but the 8566B does not have an internal tracking generator. I know there are some nice VNA type SA's on the market such as the Array Solutions 2581 and the Ten Tec/ TAPR unit. 

Being real partial to HP and Tektronix equipment, I am looking at possibly buying a 3585 A or B model. My understanding is the B model has a little better dynamic range. I have looked on the web and can't find information on the differences.
Would any of the users of this list know any differences, off line or not? Perhaps the B model uses better FET's in the front end. The A model seems to be very available and the B model a little harder to find. I am wondering if one were to buy the A model, could it be upgraded to the B by changing out a few parts?? 

Any help will be appreciated.

Regards,
Glenn WA4AOS
DSM Labs