Thank you very much for testing, indeed the NX Pro is the only reversed SBPro clone.
The NX Pro is very sensitive to system speed, it freezes the system in various games like PoP and Supaplex with anything faster than 486 (disabled L1 cache); but the SBPro does not exhibit such sensitivity.
Yes mine too is identified as an ESS, and sometimes as SBPro1 or SBPro2 with SBCHECK, don't mind that, I use it only to identify the DSP version.
The NX Pro is also sensitive about the systems noise and it varies from system to system, but all cards do.
For example on Sound Blaster 8-bit Playback Quality blog post on Nerdly Pleasures I can hear a lot of thinking noise not present on my system from the CT2230 card.
The noise on the CT2230 example is as high as the sounds themselves, I would call that unlistenable, but my CT2230 on my system is one of the quietest cards I own.
Shielding will help prevent RF noises but it will not help with "thinking noises" that are on the motherboard GROUND that reach the ISA ports, which is the origin from where most noises on a sound card come from.
Other major contributor to noise is just stock Mixer settings.
Usually the mixer settings are default at boot which is halfway on all slider on the SBPro.
Turning the Digital, FM and Master volumes of the mixer up will bring them higher than the noise floor of the system thus maximizing signal-to-noise ration.
Anyway, considering that the NX Pro is very sensitive to system speed, has reversed digital audio, and is very sensitive to noise, we can say that it is not one of the best SBPro clones to chose from.
The 2316 based Aztech cards lack a lowpass filter but they are far more successful SBPro clone.
Do you not agree?