Head and cab will be more expensive to start with but easier and less expensive to upgrade. I've always cussed at combo's when I got them because I had to "re-spend" the money I'd spent on the combo to get the thing I really needed now instead of just getting a more powerful head or an additional cabinet.
I have a very small set that I got specifically for one club we played 35 to 40-times a year. We hosted an open-mic night there on 3 to 4 Fridays a month. We'd open up playing for an hour; then let the folks who signed up come on up for open-mic; and then we'd finish off the evening after the last sign up. It was a small brew pub and I couldn't bring my DNS-410 in there, so I bought an Eden TN226 (225-watts) and a couple of Eden EX112 cabs ($269 each).
I was amazed at the great sound I got from just using one of the 112's there, so I ordered a second one. Then I started using two of them at my medium sized gigs. I made a mistake though. I bought the first one in 4 ohm so I could get all the power out of the amp. Then I bought a second one, also in 4 ohm, and had to use my "big amp" so I could run two 4 ohm cabs off the same amp. If I had purchased the 8 ohm cabs, I could run both of them from that very light Eden TN226 amp. I bought the Eden amp used for $275 and cabs were $269 new so that was $544.
Also, if I had it to do over again, I'd get the 500-watt rms version of the amp the Eden TN501. That would give me more headroom, and those cabs can handle 300-watts rms each, so as long as I don't do anything stupid, I just about couldn't hurt them.
Here's what that small set up looks like:
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I put the 4.7-lb. amp into the shoulder bag and over my shoulder, grab my bass in its case in one hand and that 30-lb. EX112 in the other hand and load out is almost finished. Was a breeze setting up and tearing down in that small brew pub. Plus, you can use the headphone jack anytime, whether or not you have the speaker cab hooked up. This by the way is normal on almost all solid state amps. It's the all-tube (all-valve) amps that can't be used without a speaker load. Doesn't matter on 99% of the solid state amps whether or not you have a cab hooked up while using your headphones.
And here's what I had to do when I used my big amp to run both of them:
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