impedance

if you have 2 cabs of 8 ohms, it makes 4 ohms, if you have 2 4 ohm cabinets it makes 2 ohms, get the 2 8 ohm cabs just so you don't fry out your amp.

 

...a cab with 400 watts rms with 100 dB/1 W/1 m and a cad with 350 watts rms with 105 db/1 W/1

The second cab is a better choice. The cab with 350 watts and 105db @ 1w has a five db advantage that the other cab can't match even with it's 50 extra watts of power handling capacity.

 

If I'm not mistaken, if you have a cab that have +5 dB more than the other, it will be nearly 4X as loud (I think+3 dB doubles volume).

So, if you are giving specific examples that apply to you, go for the cab with the better sensitivity rating and only (1) 4-ohm, or (2) 8-ohm cabs giving you a total 4-ohm load, which would match the 4-ohm head in your example.

 

No!

+10dB is double volume

To get a boost of +3dB you need twice the power, people confuse that often.

 

One more nit to pick: +10 dB is perceived as double volume.

In fact it's ten times the sound pressure and power, but we perceive it as approximately twice as loud. Damn human hearing

 

Thanks master Joris

 

Human hearing is actually pretty amazing. Probably this same phenomenon is why this particular "instrument" has a dynamic range of around 10^12 (one trillion) in pressure-squared (measured logarithmically with SPL, in decibels). That is a huge range - I can't think of many devices that have that kind of operating range.
- Mike

 

Yeah, the human hearing has yet to be equalled by equipment those humans build.

 

Puny humans!
- Mike

 

Aren't we but dust?

Oh well as long as I can play bass with my inferior equipment