First off, clipping is not your problem.
Here's a quick lesson on clipping:
Clipping occurs when you try to drive your subs with more power than the amp can deliver. The amp has a set voltage peak (called the rail), The output voltage cannot be greater than this rail. The output of a normal signal is a sine wave (remember physics class??). As the volume of the hu is turned up, the voltage from the amp increases, which in turn increases the peak value of the sine wave. Once the hu is turned up high enough, the amp will no longer be able to push the voltage up any further because it's at it's rail. If you turn the hu up some more, then the amp will try to put out more voltage, but since it's at the rail, it can't. So it cuts the top of the wave off. This is clipping. The clipping iteslf is not bad (contrary to a common belief). What's bad is the power that is delivered in a clipped signal. Remember in calculus class, taking the area under the curve? Well if you look at it that way, you have more area under the curve with a clipped signal than you do with a normal sine wave (/\/\/\/\ <--- like that) because a clipped signal begins to look like a square wave (--_--_--_ <---Like that). This is what kills the subs. It's the over powering of them. NOT sending in distortion.
What is distortion? First lets look at what a clean signal is (sine wave). An ideal sine wave is a fundamental frequency. Void of any distortion. Just a clean signal. This is what your subs "play". The THD spec of an amp tells how well the amp will reproduce that signal without distortion. The distortion is added "harmonics" or "related" frequencies of the fundamental that the amp "adds" to the signal. Therefore, an amp with a lower THD value is a "cleaner" amp. Got that? Sorry for the physics and calculus lesson here, but there is a lot of mis information in this thread that I feel compelled to correct
So, back to the clipping. When a signal clips, the signal starts to turn into a square wave. --_--_--_--_ (<------ Like that). The square wave is made up of the fundamental AND all of the harmonics. Each of the harmonics carries a little bit of power with it. When you add this all up, you get tons of power from this signal. THIS IS WHAT KILLS THE SUBS!! It's the POWER, NOT the distortion.
Now about the 2 ohm DVC. I read your post and you say that you have 2 subs. You also state that they are in series. So you have 2 subs wired to be a 4 ohm load each? Then what happens to the subs? Are they then wired together in series to give an 8 ohm load to the amp? If so, you're fine. If you ran the subs in parallel here, you'd have a 2 ohm load. If the amp can handle this in mono mode, AND the rated output of the amp is less than the rated power of the subs, then your still good. The best thing to do is put a DMM on box and find out what the actual load to the amp is. Do that and let us know. I think you may be ok since the amp does not go into protect mode. If the amp is only rated at 4 ohms mono and you're runnning it at 2 ohms mono, I'd expect it to over heat and shut down at some point. Does this ever happen?
So what is your problem? I believe the "brbrbrbr" sound you describe is probably the voice coil of the sub smacking the magnet structure. This is due to over excursion. Not clipping. So what could cause this? Well over powering the sub may make it do this, but I think your problem is even more basic than that. I think you have a leak in your sub box! Damn all this typing just for that???
Here's how to check to see if this is the problem. CAREFULLY push the cone of the sub straight down. How long does it take to come back to rest? It should take about 5 seconds or so (maybe more) to come back. If the sub comes back pretty quickly, that is a good indicator that you have a leak somewhere.
Have you taken the subs out a lot? Sometimes the seal between the subs and box can go bad with a lot of removing of the subs. Is the foam gasket on the subs in good shape? Do the subs share the same air space? Somethings to take a look at over the long weekend.
Hope this helped
Mish