Typically a higher impedance means less power. 8 ohms is the recommended impedance for full power.

Technically, you’d get around half power if you double the impedance give or take a few watts, depending on the actual design of the amplifier and the nature of the cabinet’s impedance curve. If that seems too technical, power varies with the notes played (frequency). This does not necessarily mean the 16 ohm cab won’t be louder! What counts far more than mere power, or impedance, is the sensitivity (efficiency, if you prefer) of the drivers (speakers). It’s a published spec. 3dbs difference is noticeable and requires a doubling of amplifier power to get to the same level. A 6 db difference is almost insurmountable.

When I reviewed the Jensen line for 20th Century Guitar magazine, their Jet Blackbird was far, far louder than their vintage-style p12Q. I could only do gigs with my duo act using the latter cab with my 44 Magnum. With the Jet, I could gig with my whole band! They’re both 8 ohms.

The tiny Magnum is remarkable but really comes into its own with a drive pedal that has bass and treble. Then you end up with a preamp/power amp combo on a 6″ X6″ pedal board like mine. I love walking into a gig and having guitarists in the audience ask where my “head” is. I bring a lightweight single 12″ cab with a superb high-efficiency driver and my dinky pedal board in a tiny bag. When they hear it, they shit. It blows away my Marshall JCM 60 combo–not in volume (duh) but in sheer tone. I love it.

Dear Sixties;
The most important aspect of the Plexi sound for you will be the speaker selection! Get this: put an Eminence in a Plexi stack and it’ll sound more Fendery. Put a Celestion in a Fender and it’ll sound more Marshally. I built a cabinet for my second guitarist using one Greenback and one Eminence Legend–two cheap drivers. He has a Plexi head. It sounded sweet but not as Marshally as with his original cabinet. I loved it. The Marshall sound is a combination of Celestion and Marshall. Interestingly, my Celestion Gold sounds far less Marshally than any of my other Celestions.

Marshall’s electronics do have a signature sound that varies quite a bit from model to model and year to year as you know. You’ll need a preamp with more gain than the Xotic RC Booster I use. How about a Marshall “Govner”? If you don’t have one, try a pedal–any pedal–with a British leaning. There are several available from different manufacturers and they’re promoted as such. All will work well and don’t forget: try to use one with separate bass and treble controls–not a single “tone” control–so you can fine-tune the tone to your liking. An additional EQ pedal will cut some of your guitar’s character so try to avoid that. YOU MUST TRY THE PEDAL BEFORE BUYING. My Xotic, for example, sounds great with tube-type preamp stages but pretty crappy with solid state (great with the 44 though). You must see how a pedal works with the 44 Magnum. Never buy anything recommended by anyone without trying it. We all have a different definition of good tone.
Rock on,
Magic Frank