Here are some general considerations
for making speaker choices
Cone type and bass resonance considerations:
Lead Cones (75hz): 12" M75, H75, H75-LD, J75 Scumbacks and Scumnicos.
75hz cones have a good solid bass response, mids that are even (but not scooped), with a slightly sharper treble response than the 55hz. So if your amp is a bit darker voiced, using these speakers will be one way to get more clarity out of your overall amp tone.
Bass Cones (55hz): M55, H55 Scumbacks
55hz cones handle deeper bass notes more easily than other cones, have strong lower mids and a creamier high end treble response. This might be why Jimi Hendrix is said to have used them later on in his career, which I believe is what was used from 1968 on. Notice how fluid his tone is in the Band of Gypsy's album recorded at the Fillmore East on New Year's Eve, 1970, for example.
Dust caps make a tone difference. Most 12" speakers use a small dust cap 2 1/4" wide, but the large 4" wide dust cap (LD or G12-65 type) diffuses the highs a bit more, making it great for single coil guitars that have enough low end, but are bright on the treble side, or those who play with high gain (very hard rock/metal players), pedals & fx, etc. It also slightly lowers the bass response, so think of the LD model sort of like a 65 hz bass response speaker, sitting right between the 55 & 75 hz models.
Magnet Weight Influences:
Generally speaking, the heavier the ceramic or alnico magnet, the more sensitive the speaker, and the louder it gets (i.e. more efficient), usually.
M weight magnets, 35 oz. (typically). These magnets usually produce a strong midrange output around 97 db with a 75hz cone, 96db with a 55hz cone. Same magnet weight as the Pre Rola G12M.
H weight magnets, 50 oz. (typically). These magnets usually produce a more balanced tonal output around 99/100 db with a 75hz cone, 98/99db with a 55hz cone. Same magnet weight as the Pre Rola G12H30. An H speaker is about 15-20% louder than an M speaker with the same amp.
Guitar & amp characteristics:
Please understand one thing. Most amps are rated for their advertised RMS wattage at clean signals before the edge of breakup. On most amps that's going to be at 3-4 on the volume knob. Your typical old Marshall Plexi circuit 100 watt amp head (1959, Super Lead/Bass amps) puts out 120 watts at 4 on the volume knob. But you like to play it at 7, right? :-) If your amp is healthy, around 7 on the volume knob it puts out 180-190 watts (or more). The guys at Marshall knew what they were doing when they put the "100" logo in the corner of the cabs, but that still meant that you needed two of them to handle a 100 watt amp that put out 180 watts at the levels you set the amp! Please get your amp tested for it's real output wattage, because no one likes the smell of a melted speaker, OK? Always figure you need twice your amp's rated RMS wattage in speaker power handling to safely run your amp on 10.
If you get two 30 watt speakers to run with your 50 watt amp, and dime it, they'll be good to go for awhile. I don't know what timeframe "awhile" is. I've safely done it for 2 hours straight, without issues. I have not done that for extended periods, so you want to take that into consideration with all the OD pedals, clean boosts, and so forth that push speakers to raise your level for solos. Use your good old common sense here. Sure, your old small block Chevy/Ford sounded great and was running awesome at 8000 rpm for "awhile", then the crank, piston, or connecting rod broke through the block, and you weren't mobile anymore. Use the same common sense judgment with choosing your speaker wattage.
Speaker Break-In Timeframe
If you are thinking that you're in tonal bliss right now, just wait. You'll be more surprised in 20-50 hours of break in time at stage volumes (loud enough to be heard over a drummer). All Scumbacks are built to exacting specs like the originals from back in the 60/70's. Scumback cones are aged to sound old, and many have reported that they sound identical to Pre Rola G12M and G12H's from the 60's after minimal break in time with the low power models. Just to be clear on that, though, I've asked several of my clients to report what they experienced. So here's their comments boiled down to "how many hours?" for break in time...
"20 to 30 watt speakers generally sound their fullest and most articulate after 20-25 hours of break in. The louder you play them, the faster they break in by the way, because it moves the suspension & cone more, making them more flexible and more dynamic sounding. As your speakers break in they will get more defined, more mids, rounder bass response and their "voice" sounds louder."
"65 watt (HP model) speakers generally take about twice as long, so figure 40-50 hours before they realize their best tone, with the same attributes as the lower wattage models."
"100 watt (XHP) models sound virtually the same as the 65 watt versions, but they need 60 hours to fully break in due to their higher temperature voice coils."
Many have asked me the differences between the lower wattage, 65 watt HP, and 100 watt XHP speakers. The higher wattage voice coil exhibits a tiny bit more bass response & a bit less treble response. That's if you're really listening and recording and comparing tones. In my tests, moving your bass control from 7 to 5 usually takes care of the difference between the low power and either the 65 or 100 watt versions. The treble might need to be moved from 5 to 7 to make up the difference. They all sound fantastic, so order the speaker power handling you need to handle the ouput wattage of your amp where you play it, and wail on them. That's what they're made for, after all!
Assess Your AMP Tone Judiciously!
Assess your amp tone. If you think it needs help, consider all of the factors involved before you get the signal to the preamp section of your amp.
- Preamp tubes can make a difference, and are a cheap way to experiment with gain and tone.
- Power tubes can make a difference, but to a lesser degree, in my experience than preamp tubes.
- Your guitar pickups & wiring are a factor, as well as pots & caps. Better caps & pots sound better.
- String gauge, the thinner your gauge, the thinner your tone unless you have a lot of amp behind you.
- Check your FX pedals, cables, and they're health, as one bad cable can make you sound thin/noisy, etc.
Hopefully this will give you a guideline on choosing speakers for your needs that will give you the tone you're looking for and you can run safely.