Speaker advice for Silver Face Bassman

[revolver]

I've just acquired a mid-70's Bassman 100, which came with a slightly later (late 70's / early 80's) ported Bassman 115 cab.  The amp requires a 4ohm load, but the cab is 8ohm (I think it would originally have paired with the Bassman 135, which I believe had selectable impedance?)

 

The previous owner appears to have been using this combination for a good few years, but I've variously read that valve amps should be properly impedance matched, but also that Fenders are quite forgiving in that regard.

 

So my question is, should I be looking at getting a properly matched load at the earliest opportunity and, if so, is it worth trying to replace the existing speaker with an appropriate 4ohm equivalent?  Or has cabinet technology advanced so far in the last 45 years that I'd just be better off considering replacing the cab entirely?

 

Cheers!