All speakers have what is known as 'free air resonance' (Rs). In an open back cabinet, there will be a lot of bass output at the the resonant frequency quoted by the maker... usually 75-85Hz for a typical 12" guitar speaker. Big open back Fender combos, and other makes, display silly amounts of bass at this frequency... open E (6th)... because the speaker naturally vibrates heavily at this frequency, even with only small amounts of power driving it.
The same speaker(s) in a closed back cabinet will perform differently. The back on the cabinet seals the air inside the cabinet. So the speaker has to work harder to compress and stretch that trapped air. This places stress on the speaker cone and causes the resonant frequency of the speaker to change. The sound also only comes from the front of the cabinet, making it appear directional, as there's no 'spill' from the back of the cab.
Regardless of the size or shape of the closed back cabinet, the resonance of the speaker will rise by about 2½ notes. This means that most closed back cabinets (even a Marshall 4x12) have a resonance of approximately 120Hz... or open A (5th). Great for rock!!
If you're a blues, country or jazz player, then the closed back cab may exude too much chuggy open A bottom end. You can't turn it off.
If you find that an open back cab is still too bassy for you, then you must replace the speaker with one where the resonance is down at 55Hz... that's much ower than open E. The Celestion G12H 30 Heritage does just this job. So does the JBL D120F or the Electrovoice EVM12L. They all have very supple cone suspensions which facilitate this low resonance effect.
Larry Carlton's ES335 tone with the Crusaders is a good example of the EVM12L 55Hz sound. However, for some rock players it may sound a little flat and lifeless.
10" speakers have a higher Rs around 100-120Hz, but the same effects will apply... just raise the Rs by 2½ notes for a closed cabinet.
