A single recto is rated at 50watts RMS
50watts into a 120watt cab will sound fine and you won't blow the speakers.
a 2 x 12 rectocab has two Celestion Vintage 30 speakers in it, both of which are capable of putting out 100db / watt / metre. Basically, one metre away from the speakers, they generate 100dbs with a watt running through them. I guess you'd end up with 103db at a metre with two watts, 104.5 with three watts etc.
100db/watt/metre is very loud for a guitar speaker. Some guitar speakers put out 97db / watt / metre which requires more power from a head to get the same volume. Another common Celestion driver, a g12T -75, puts out 97 DB / Watt / Metre. So a Marshall 2 x 12 cab with these speakers will handle 150watts without blowing BUT it puts out less VOLUME than a 120watt 2 x 12 loaded with v30s. The 120 2 x 12 can handle less power going through it, but puts out more VOLUME per watt. So, our 120watt rectocab will be as loud with a 50watt single recto as our Marshall 2 x 12 150watt with a 100watt dual recto.
a 4 x 12 will have more presence because it moves more air. A 1 x 12 will have less presence because it moves less air.
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