The SP1000 and SP1000M sound exactly the same as far as I can tell.
On paper the SP1000M technically outperforms the SP1000 in mode for SNR, crosstalk, and THD+N when both units are in balanced mode, and matches it in everything except THD+N with both units in single-ended mode. And the SP1000M has better clock/jitter performance.
The differences are below audible limits, however, so they’re don’t really mean anything.
I like the Stellia best out of the N8 in tube mode.
But then I like almost everything better out of the N8’s tube mode vs. any other DAP or the N8’s solid-state outputs. Which has little to do with technical fidelity and more to do with just raw enjoyment.
In other words, solid-state mode out of either player is going to be technically higher in raw fidelity if not necessarily as enjoyable to listen to (that’s always going to be personal) … though both players sound amazing regardless of which output or mode they’re in.