Tube amps are safest long-term with an exact impedance match. I won't risk my trannies.
I used to think that of the two, only mismatching with a higher impedance cab was safe (although risky long-term). But people also write that the other way, that mismatching with a lower impedance cab, is safe! There's proponents for both camps, each with
experts (and technical info!) to back them up. The result is a flurry of debates about the inner workings of current.
Thing is, the previous owner of my amp routinely ran 4ohms into an 8ohm cab. For a while, I ran it that way too. More robust amps will handle that mismatch, but I understood that there is a risk. Now that I got everything matched up, I don't think there's a significant change in tone or response, to warrant a possible long-term risk by me going back to a mismatched amp/cab setup.
I dunno about you, but will you risk it? If that amp you have breaks down, can you afford another one like it? And for that speaker distortion you want, you can have that by closely matching
wattage with your amp/cab. That warrants another topic though.