Cone type and bass resonance considerations:

Lead Cones (75hz): (Bold) M75, H75, H75-LHDC, SA10-H75 (10") Scumbacks

75hz cones have a good solid bass response, mids that are even (but not scooped), with with a slightly sharper treble response than the 55hz. So if your amp is a bit darker voiced, using these speakers might be the right way to get more clarity out of your overall amp tone.

Bass Cones (55hz): (Bold) M55, H55 Scumbacks

55hz cones handle deeper bass notes more easily than other cones, have strong lower mids and a creamier high end response. This might be why Jimi Hendrix is said to have moved to 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:

Dust caps make a tone difference. Most 12" speakers use a small dust cap, but the large dust cap (LHDC 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 LHDC model as the 65 hz 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. The neodymium type magnets are lighter and more powerful, but they don't exhibit the "old tone" of the 60's, so they're not my favorite sounding speaker.

M weight magnets, 35 oz. (typically). These magnets usually produce an 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 an output around/100 db with a 75hz cone, 98/99db with a 55hz cone. Same magnet weight as the Pre Rola G12H. An H speaker is about 30-50% louder than an M speaker with the same amp.

Guitar & amp characteristics:

Humbucker guitars put out more low end than single coil guitars, and sound fatter (generally). They work well with M75, H75, H75-LHDC and H55 speakers (both regular and HP models that handle 65 watts).

Single coil guitars have more high end than humbuckers, but not as much low end (beef). They can also sound thinner overall with less midrange (scooped sounding).

Popular Amp & Speaker Combinations

Marshall Plexi JTM 45's (either KT-66 or EL-34 models) have a lot of bass response and put out 30 watts tube rectified, 50 solid state rectified, so the popular choices have been as follows:

M75's in a closed back cab, H75/M75 mixed in closed or open back cab, H75/H55 mixed in closed and open back, as well as the H75-LHDC. The H55 speaker has so much low end that I don't recommend two or four of these together as the only speakers used, I will always recommend an H75/H55 mix in a 2x12 or 4x12. There is too much of a good thing, and four H55's qualify as that. I personally use the H75/H55 mix in both my open back 2x12 combo and 2x12 adjustable back cab, as well as my 4x12 demo cabs. Why would you mix speakers? Because it fattens up the midrange, and low end, or it can produce a particular breakup characteristic you like at different volume ranges.

Marshall 100 watter amps: H75's in a 4x12, M75's in two 4x12's, H55/H75 mixed, and H75-LHDC models.

Two Rock/Fuchs/Dumble amps: H75-8HP/ 16 HP speakers for 1x12 or 2x12 cabs. The H55-HP and H75-HP mix in a 2x12 vertical or horizontal cab has been very popular with these amps, as well as the Mad Professor users.

Fender amps: H75 and H55 for open back combos (Deluxe, Princeton, Vibrolux). If the combo cab is shallow, the H55 will add to it's low end response. If it's a bit deeper (more than 10") then the H75 is a good choice.

Marshall 18 watt style combo amps benefit the most from the H75 models, unless a closed back cab is used with an 18 watt head, then M75, H75 and H75-LHDC models have been very good. 2x12 combos & cabs work with speaker mixes, too.

AMP Wattage!

Please understand one thing. Most amps are rated for their advertised RMS wattage at clean signals. On most amps that's going to be at 2 on the volume knob. So your 100 watt Marshall puts out 100 watts at 2. You like to play it at 7, and if your amp is healthy, that's when it puts out 150-180 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! Get your amp tested for it's real output wattage, no one likes the smell of a melted speaker, OK? Always figure you need twice your 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 sounded great and was running awesome at 8000 rpm for "awhile", then the crank, piston, or connecting rod broke, 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, wait a bit, you'll be more surprised in 20-50 hours of break in time. All Scumbacks are pre-aged, and slightly broken in when they're manufactured. Each Scumback speaker takes five days to build, and it's not just slapping the cone in a frame, centering it and shipping it out, either. Scumback cones are aged to sound 25 years 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. 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...

"25 & 30 watt speakers generally sound their fullest and most articulate after 15-20 hours of break in. The louder you play them, the faster they break in by the way, because it moves the voice coil/cone more. Reports indicate they get more defined, more mids, rounder bass response and 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."

Many have asked me the differences between the lower wattage and HP speakers. Easy. The larger 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. They both sound fantastic so order the speakers you need to handle the wattage of your amp, 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.

1. Preamp tubes can make a difference, and are a cheap way to experiment.
2. Power tubes can make a difference, to a lesser degree, in my experience.
3. Your guitar pickups & wiring are a factor, as well as pots & caps.
4. String gauge, the thinner your gauge, the thinner your tone unless you have a lot of amp behind you.
5. 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.