The combo, as I'm sure you are aware, is 50 watts. That other cab on which is sitting the Herbert and Einstein combo, is a one-off Diezel stereo studio cab with 2 G12-H's in it that I use at times with the Einstein combo. It can get really LOUD.
The amp is more then loud enough for any situation I have ever played in by itself. I play in a 5-6 piece band and it can get loud on stage. I don't play shred or head-banging music, and I have changed out the preamp tubes to 12AT7's and 12AU7's , which lower the overall output volume of the amp as a combo. The power tubes I use are JJ KT77's and the amp sounds great with them! (If you leave the stock preamp tubes in the combo, it will anihilate your ears! It is really LOUD!)
Still, I have the masters on no more then 2 o'clock and the amp kills for me. I have a stock V-30 Celestion, which came in the amp. While these are not my choice, but Peter's, I have left it in, but use a Weber beam blocker, which cuts the harsher highs and focused sound eminating from the center of the cone. It sounds great now and disperses the sound all over the stage.
What is nice about using the combo alone, is the fact that you can turn it up more then with a 2x12, or 4x12 and work the snot out of the tubes. This amp is just terrific, imho.
If you need more volume, simply bring out another 1x12 or 2x12. Also, which I like, is the fact that the extra cab is 16 ohms run on one cable alone, or 8 ohms when running each speaker in the 2x12 separately with it's own speaker cable to the head with the combo, which has a 16 ohm Celestion in it. The mismatched ohmage brings out a great and unique tone to the amp.
I also learned this trick...If I am running the combo and a stereo 2x12, and use each speaker input separately in the 2x12 along with the speaker in the combo, the sound gets louder yet! This is what I meant by running the speakers in the 2x12 separately. This happens even with a head and running the cab with both speakers separately, as opposed to just using one speaker input on the cab to the head.
This technique really helps me get just that much more volume from the Schmidt then running only one speaker cable to the Schmidt head alone. The difference in volume is quite noticable.
If you don't play at extreme volume levels such as using a full or half stack with 100+ watt heads turned up really loud, you should really enjoy the Einstein combo. I find it to be an extremely versatile amp combo on it's own, but with extra speakers, it can stand with the heavier amps.