Why not buy something very good from scratch and that's it?
The problem is that everyone hears a little differently. A system that sounds great to you might be disappointing to someone else and vice versa. There is also system synergy to consider. Depending on other components (such as your source, preamp, cables), one amp may sound better than another. These are the main reasons people tend to try multiple products.
That said, you're more likely to find a good match by starting with a well regarded product. Measurements only tell part of the story. An amplifier that measures really badly will almost certainly sound worse than an amp that measures great, but keep in mind that traditional measurements use one or a few pure tones into test loads. They don't measure performance on real music into real speaker loads. And the parameters that are measured don't always correlate well with listener's preferences.
As an example, very highly regarded Pass amplifiers have considerably worse measurements than an amp like the Neurochrome Modulus amps, but many audiophiles strongly prefer the sound and are willing to pay many times more money. The same can be said for most tube amplifiers.
If you want build your own amplifier, and particularly if you don't have a lot of DIY electronics experience, I think the Neurochrome amps are an excellent place to start. Since the amplifier module itself is already assembled, the full amplifier is fairly easy to build and your results are likely to be consistent with other builds. These amps are very clean sounding and fairly neutral, and the Mod-286 provides enough power to drive the NX-Oticas fairly well, particularly when supplemented with the powered subs. For the money required to build them, I doubt you will find anything that will objectively perform any better.
If you can afford it, I'd recommend a dual-mono build (meaning separate transformers and power supplies for each channel) or monoblocks. I haven't personally compared the single supply build of these amps with dual mono, but in my experience with many other amplifiers, separate power supplies provide better imaging and dynamics (sometimes significantly better) than the the same amp built as a single-supply stereo amp.
I currently have a pair of Neurochrome Mod-286 monoblocks in my amplifier rotation used to drive my NX-Oticas and find the sound to be quite good. Is it as good as a $5K or higher amp from Pass, Parasound, Bryston, etc.? Probably not, but it is close in my system. Fortunately, the NX-Oticas are fairly efficient and easy to drive.
But I'll reiterate my first point. Everyone hears a bit differently and will appreciate different aspects of the sound. And system synergy is key. I don't think you will be disappointed at all with the NX-Oticas and subs, but if you have the opportunity to try different electronics and cables, I'd encourage you to do that.