Just checked it on +2A myself... looks funky indeed Ideally it should look more like this on 48K, too - then sound would be cleaner. I've got some ideas to improve sound by always keeping the beeper afloat rather than shutting it off at volume 0. But it'll be quite some work to implement, and I'll have to reduce the mixing rate, though there's some room for that.
This is great, love the engine and the border FX 
BTW, I've just tested the demo-nmos.tap file on different speccy models using Spectaculator and get very different border FX on different models 48K, 128K and 128+2A
The FX on the +2A are brilliant
fluidcore is a 4 channel PCM wavetable player for the ZX Spectrum beeper, using looped 256 byte waveforms. It offers a total of 17 volume levels, and can handle up to ~860% overdrive when the maximum volume level is exceeded. Sound is mixed at approximately 23 KHz.
Needless to say, I'm pretty proud of this thing Also, the good news is that I've managed to make an XM converter for it. So, hope you'll enjoy!
download (includes XM converter, pasmo is required)
source code
demo video
48K hardware recording