It's not any "tougher" to drive than the 2 Ohm version. Their rated sensitivity is the same. The problem, if you want to call it that, is that there are VERY few amplifiers capable of producing 1000 watts or more (which the Ultimo really needs for optimum performance in the lower frequencies) with a 4 Ohm load. Most amps are able to do that at 2 Ohms, or 1 Ohm. Thus, if you have the 4 Ohm version of the Ultimo, you will have fewer options available to you for amplification that can properly drive the Ultimo. You can get by with less power (and I have used it with less), but trust me, you will not be hearing the full capabilities of this sub. Put on large scale orchestral music and you will hear what I mean. The downside to operating your amp with a 2 Ohm load is more heat and current requirements. So you better make sure your battery and alternator are up to the task.I've heard (from Morel) that the 4 ohm Ultimo 12 is very tough to drive. I was interested in a 2 ohm model.