Sorry, but the same questions keep coming up, and the same wrong answers keep circulating, so here goes:
Yes, you will get some drive with just an adapter cable, but you won't get the full power output or the best sound in most cases.
The headphone out of an iDevice won't drive a bass cab to even satisfactory bedroom levels without an external amplifier. A regular DI (active or passive) has no gain, and requires an external preamp as well as a power amplifier in order to interface between an iPhone or iPad and a bass cab.
If you have a pro power amp without a 1/4" unbalanced input, an adapter cable won't be enough. You'll need at least a DI, an audio interface, a small mixer, or a mic pre that has a balanced XLR output.
In most cases, the sensitivity of the power amp will be pretty low, and you'll need more gain than a DI or even a lot of preamp pedals can provide. The situation is better if you're going into the effects return of a bass amp or into a powered speaker, but you will likely still need some gain from an active device.
Good budget choices are:
- ART Tube MP mic preamp (cheap 1-channel tube preamp, all analog, all the gain you could need, also makes a great bass DI)
- Mackie 402VLZ3 (compact stereo mixer, balanced and unbalanced ins and outs, EQ, crazy-good preamps, lots of gain)
If we're talking an iPad rather than an iPhone, another option is the ART USB Dual Pre. It's an actual digital audio interface, requires Apple camera Connection kit, but it does A/D and D/A outside the iDevice. Other class-compliant USB audio interfaces will work as well, but compatibility is a bit spotty, and none of them will work with an iPhone.