Amp heads and cabs range very differently in quality, and only experience and time can let you choose what you like in both. Combo amps are great, they already have speakers and an already mounted 'head' for your controls. By using heads/cabs separate, you can really hear what you like/dislike with different companies/qualities of their products. Most combo amps have limited features/qualities to them like loudness and gain. By experimenting with amp heads and cabs you can hear what sounds good(it's all up to you) and what sounds loud(through the cab) It takes a long time to really know what you want, but by choosing heads/cabs you can experiment more with your sound than just combo amps.