Re: Anyone tell me about a Fender Bassman head?
Yes, you can a/b your heads with an amp switcher. Radial Tonebone makes one, called the Headbone, and Mesa and CAE also make them. Radial makes several cool products that you should look at.
Yes the Bassman is wired for a 4 ohm load, but you may be able to change the wiring of your Marshall cab to make it a four ohm cab.
A mid-60s BF Bassman is a sweet amp for guitar, and many folks have used them over the years. The 6L6s give a little "airier" tone with an extended trebel and bass response that won't frequency limit as much as the EL34s that Marshall uses. In general, I would avoid the later SF models and the 100-watt versions, as they are less guitar friendly.
Bill