building the X is a bit more difficult tham building the aleph 3 but I think it is worth it.
There are a few descriptions of the difference in sound between X and Alephon the forum. Here are two I found but there must be more:
I agree with kilowattski. The Aleph-X can be customized to suit almost any impedance load. It depends mostly on the number of ouput devices used and the bias current level, although most of us have chosen to optimize our Aleph-X's for operation into 4-8 ohm loads.
I'm always amused and amazed by how these rumors get started. Either the Aleph or the Aleph-X will do just fine with low-Z loads, assuming sufficient bias.
When I first started the original thread on the Mini-A, just about the first thing people asked was whether it would handle a 4 ohm load. In that case, no, I had not provided enough bias for 4 ohms, although increasing the bias is a pretty simple operation. So when I did the Aleph-X I made sure to boost the current so that it would (almost) handle 4 ohms. If you want more bias still, have at it. Just make sure that you have a robust power supply.
as i recall (which could be incorrect), you can make an alephx that can be well suited for 4 ohm loads, but it would be HIGHLY inefficient and produce massive amounts of heat and have a very robust power supply. i just remember its better suited with 8 ohm loads.
just about any amp can be customized to work at different load ratings, some are just more suited for different applications.
Any amp biased heavily enough for a low-Z load is going to be less efficient, regardless of topology, gain devices, etc. The lower the load impedance, the higher the bias required, and the greater the heat generated at idle. If it's not used to produce sound, it goes up as heat, making the amp inefficient. You can't beat physics. All you can do is back off and go class AB or D or something, but then you're changing the rules of the game.
An amp becomes inefficient if you want to be able to drive both low and high impedence loads. The efficiencies for low impedence loads can be improved by reducing the rail voltage, but the amp will not be have enough voltage to drive high impedence loads.