My system consists of the following:
2005 Nissan Altima SE-R:
Source: Kenwood Excelon DNX9960
Line Driver: Tru Tech SSLD6i
Front: Dynaudio System 360
Front Amp: Pioneer GM-X1022
Rear doors: Blaupunkt 8" Triaxial in rear doors
Rear Deck: 10" Pioneer TS-W253F Diamond plates (2)
Rear/Sub amp: Massive NX5 powering rear doors and subs.
So I got one of the NX4's because I thought it would be perfect for my wifes car in terms of power output and size. I was right about the size. I also have had an NX5 for quite some time.
First and foremost the NX5 - The sub section isn't bad, I've got it running at 2 ohms on a pair of 10's IB. Obviously they don't need that much power to get going, so that is the caveat to that. The 4 channel section of that amp is overrated IMO. in bridged mode it should be putting out 240 rms, but my 100x2 rms amp up front (Pioneer GM-X1022) blows it away in output. I expected more, much more. I was at some point considering going with just that one amp, but quickly decided against that after having run it a while. Also the output from the rear door triaxials is pretty muddy. I would not run one of these amps on a front stage where clarity is the focus. This amp will stay in my car for 3 reasons, size, sub power is adequate for my needs, no cash to replace it, though I may stick a pdx 4.150 and pdx1.1000 in place of it to see how it turns out.
Wifes Car - 2009 Toyota Corolla:
Source: Pioneer AVIC-Z110bt
Front: Eclipse SC8365 3-way comps with midrange and tweet in A pillars
Front Amp: (Was NX4) Now PDX-5
Rear Deck: DLS OA8's pair in rear deck
Rear/Sub amp: (Was NX4) Now PDX-5
Next the NX4. This amp, IMO is extremely overrated as well, and here's why. The amp should be able to do 100x2 rms and 400x1 rms bridged when run in 3 channel mode. Great! Well the output on the components in my wifes car up front was, to say the least, very unnatural sounding. Overwhelming shrill output, and the amp seemed to put out quite a bit less power than I anticipated. (I'll give more reasons for my thinking this in a few lines down). I did not put the amps on the bench and I have no o'scope, so this is simply my ear telling me what I heard.
On the sub side, the amp REALLY disappointed me. The OA8's were the second set I owned, I had a pair in my Altima way back and they sounded very good. With the NX4 the subs were extremely dirty sounding with poor output. I had put the subs in and decided that if they didn't sound good I would replace them with something else (that's when I bought the eD 8's in the hot deals section). I just didn't like how dirty they sounded and how weak the output was for being rated at 400x1 rms, it sure didn't sound like it. Another thing I noticed with the NX4 was quite a high noise floor and alt whine.
So the next step was to either put in the eD's - which I waited on doing, and also put in a PDX-5 I had waiting in the wings that I just bought a couple weeks ago. I decided to start by putting in the PDX-5. Immediately the subs output was cleaner, clearer, and much much more intense. The front stage was a totally different animal off the PDX-5, it sounded much cleaner and clearer as well. The amp was initially installed with just channels 1 and 2 on the fronts, then I bridged it and was even more impressed. The PDX-5 does around 214x2 bridged (107x2 rms on birthsheet) and 458 rms on the sub. On both, the PDX-5 rips the NX4 apart. I was thoroughly impressed with the non bridged output, and again once I bridged the amp it was even better. The last thing I noted - alternator whine GONE and noise floor substantially lower, but still audible.
Here's my review summed up - Overrated, over-hyped, poor sound quality amps.
Disclaimer - these are my thoughts and experience with these products, yours may vary. I do not claim to be any kind of expert, I just rely on my past experience and product usage to arrive at these conclusions. This review is not intended to skew anyone away from buying this amp or not buying this amp. Solely my opinion and nothing more.