Hey Rick,
First, let me say that you'll probably get lots of different recommendations, most based off personal preferences, but you'll have to keep in mind your intended SPL/volume as well as the tone you are looking for. To that end, I won't throw brand names into the mix.
The
ABM 900 puts out two channels of 575W at 4 Ohms, or at least I have to assume the 4 Ohms since the specs and manual are vague on that point. This also means that at 8 Ohms, you'll likely see around 350W per channel.
Depending on which Randall he has, the 2x12 is only rated for 120W. If you used that with your head, you'd keep turning the volume up to get the sound you need, until things start melting or farting out. The speakers and cabinet are also not designed to produce bass frequencies at the volumes you want. Not recommended at all.
As for speaker size (10, 12, 15, 18, whatever), that's become a matter of preference. With neodymium drivers, the performance is generally about the same with each driver. For ceramic drivers, the size does play a role as things are generally heavier and slower.
If you use distortion, cabinets with a dedicated midrange (and/or tweeter) will let you keep that clarity even when things are getting dirty.
After that, it comes down to how much rig you want to tote around, and whether the rig will be used for just stage monitoring (just for you and the band, DI to the crowd), or sound reinforcement (heard by the audience).
Dekker
