Hi Crion,

I don't think the 218 ever had a linear power supply, it simply would not fit. What is has is a cosel TUHS5F12 SMPS. It cost about 15euro and does the job power efficiently. I think the circuits are advanced and changing the power supply gives the performance a giant step.
Remarkable that M mentioned a linear power supply. Even more that there is no upgrade path for it, would have been really easy and they could have sold PSU's. Maybe they were worried about the sales of 818v3? Which is probably even better.

On the installation of the socket; the space that comes free from removing the AC socket can be covered with a small plate on both sides, clamping it with small bolts an holding the 2.1 DC socket. I haven never seen a 210 from the inside, but it probably has a similar PSU.
Regards

Main: DSP8000SE< 218 < Farad Super3 TMCB < PS Audio P10. Room correction: Focus Fidelity.
Second: Fostex Fe206NV2 < Audio innovations 500 valve amp < Wiim Pro +
Study: DSP33 < Reviver < MS200
Server: ROCK + Tidal on NUC, fiber on 2x cisco 2960, HD-Plex LPS.
Gone: DSP7200SEu, DSW, DSP5500mkI & DSP5000mkI, 508.20, 210