Ok, I am aggregating a few of my posts (with edits for "brevity," lol)...
Since 1963, I have owned or used extensively 12 or 13 combos, but who's counting? I don't have one now, but wouldn't rule it out. There is nothing inherently evil or bad about combo amps, but besides the advantages of supposed convenience, some sacrifices in quality and/or performance for a budget price are not uncommon. Some are very light, and some are very heavy. Some sound great, some don't. Some have extension speaker outputs, some don't.
It's hard to generalize about combo performance, price, weight, etc., except to say you will usually get what you pay for (buy used!), and that to upgrade performance to the level of a typical good modular rig may require lugging two combos, or a combo plus a powered cabinet, or perhaps just the combo with an extension speaker. These extra, possibly large boxes, all make combo ownership for price and convenience less compelling, as it is now a modular rig with the head trapped in a cabinet or two.
Thus, I'm re-introducing the epithet "combo trap." The combo trap is composed of problems such as, if the combo amp dies, the speaker is useless. Or, if the combo speaker dies, the amp is either useless until the whole thing comes back from repair—or you get to lug the whole combo with a dead speaker as the amp head for a different cabinet. All of these have happened to me.
Oh BUT, the (theoretical) combo lover will tell you "my combo is soooo much easier," and yes, if plugging in a speaker cable is too hard, your life will be better, I guess. Theoretical combo fans also say "moving a combo is easier," which is fine, if it is actually true.
Many combos are huge and heavy, though new lighter ones are more portable than the current ~70 lb. Hartke combos or the old combos like the 74 lb. SWR Workingman's 15 or 78 lb. Acoustic 136 I used to schlep. A combo's weight is always a big selling point, so many are very light, but all combine a speaker cabinet with a head. For it to be super light and cheap means trade offs somewhere. Care to guess where?
Let's say you want out. How to go modular when all you have is a single combo amp? Assuming your combo has a external speaker output, adding a good quality used cabinet can be a bridge for escaping "combo trap island." Get an extension cab that is a lot better than the built-in cab/speaker. After using the better cabinet for a while with the combo, you might want to simply move to a higher power micro amp with the better cabinet. That is, if your funds are not all tied up in the combo + cab.
Selling the combo to raise funds for a head is a problem if it won't yield enough profit to buy a nice USED head, so save up first (part of the trap thing). Keeping the combo for smaller gigs and as a spare or reheasal rig is a good idea, too. IME, selling a combo that was purchased new will not likely yield enough funds for new "upgrade" head. Part of the trap.
If you go modular, with a separate amp and cabinet, you can often actually get something more portable than if you simply add a extension cabinet to your combo. The newer micro heads deliver the goods while weighing in ~5 lbs. A good 112, 115, or 210 neodymium driver equipped cabinet can weigh between 25 and 35 lbs. Slip a micro head like a GK mb800 into your gig bag, and the cabinet will probably be lighter than your current combo.
My 212, a fEARful 12/12/6/1, has tilt back wheels, and is a breeze for this 70 year-old geezer to move and load and in/out of a vehicle. I don't have to carry the head at the same time, but I can, as it is in a shoulder bag.
Getting a rig that has a separate head and cabinet allows you to add a cabinet, upgrade the cab(s), replace the head, and sell either part if it becomes redundant. Modular rigs are free of the "combo trap" problems. You can even make the head itself modular, with a power amp, and a pre-amp (or two) in a rack.
But remember, nothing lasts forever. Be prepared and have a spare. I always gig with a spare compact head in my trick bag.
